Entity Central Index Key Interest bearing deposit brokered. Time deposits less than 250000. Time deposits $250,000 or more. Time deposits brokered. Income tax reconciliation bank owned life insurance. Deferred tax assets liabilities before valuation allowance gross. Collateral dependent impaired loans fair value. Real estate acquired through foreclosure fair value. Gain loss on sale of collateral dependent impaired loans. Gains losses on assets held mortgage servicing rights. Gains losses on assets held foreclosed real estate. Merger and integration cost. Segment other noninterest expense. Income (loss) before income taxes and elimination of intersegment profit. Elimination of intersegment profit. Customer service fees. Gain on loan origination and sale activities net excluding intercompany profit. Mortgage servicing revenue expense. Segment other noninterest income. Mortgage servicing rights. Employee stock ownership plan loan. Mortgagors escrow accounts. Income (loss) from continuing operations, net of tax, before equity method investments. Impairment of foreclosed real estate. Increase decrease in mortgage servicing rights. Increase (decrease) in intercompany receivable. Payments for proceeds from acquiring certificate of deposit. Proceeds from pay downs of mortgage backed securities available for sale. Proceeds from sale of portfolio loans. Proceeds from employee stock ownership plans loans, investing activities. Net increase in non-brokered deposits. Net increase (decrease) in brokered deposits. Increase (decrease) in long term loans to federal home loan banks. Increase (decrease) in mortgagors' escrow accounts, net. Stock repurchase payable. Gain on loan origination and sale activities, net. Debt and equity securities gain loss and impairment write down. Amount of revenue earned, classified as other, excluding interest income. Amount of noninterest expense classified as other. Cover [Abstract] Document Type Document Type Amendment Flag Amendment Flag Document Period End Date Document Period End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Trading Symbol Trading Symbol Entity Registrant Name Entity Registrant Name Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Entity Voluntary Filers Entity Voluntary Filers Entity Well-known Seasoned Issuer Entity Well Known Seasoned Issuer Entity Filer Category Entity Filer Category Entity Current Reporting Status Entity Current Reporting Status Entity Small Business Entity Small Business Entity Emerging Growth Company Entity Emerging Growth Company Entity Ex Transition Period Entity Ex Transition Period Security Exchange Name Security Exchange Name Title of 12(b) Security Security12b Title Entity Incorporation, State or Country Code Entity Incorporation State Country Code Entity Interactive Data Current Entity Interactive Data Current Entity Common Stock, Shares Outstanding Entity Common Stock Shares Outstanding Entity Shell Company Entity Shell Company Entity File Number Entity File Number Entity Tax Identification Number Entity Tax Identification Number Entity Address, Address Line One Entity Address Address Line1 Entity Address, City or Town Entity Address City Or Town Entity Address, State or Province Entity Address State Or Province Entity Address, Postal Zip Code Entity Address Postal Zip Code City Area Code City Area Code Local Phone Number Local Phone Number Document Annual Report Document Annual Report Document Transition Report Document Transition Report Entity Public Float Entity Public Float Documents Incorporated by Reference Documents Incorporated By Reference [Text Block] Statement Of Financial Position [Abstract] Assets Assets [Abstract] Cash and due from banks Cash And Due From Banks Interest-bearing deposits Interest Bearing Deposits In Banks Total cash and cash equivalents Cash And Cash Equivalents At Carrying Value Certificates of deposit Certificates Of Deposit At Carrying Value Securities available for sale, at fair value Available For Sale Securities Loans held for sale, at fair value Loans Receivable Held For Sale Net Not Part Of Disposal Group Loans, net of allowance for loan losses of $4,280 in 2019 and $4,437 in 2018 Loans And Leases Receivable Net Reported Amount Federal Home Loan Bank of Boston stock, at cost Federal Home Loan Bank Stock Accrued interest receivable Accrued Investment Income Receivable Mortgage servicing rights, net Mortgage Servicing Rights Premises and equipment, net Property Plant And Equipment Net Bank-owned life insurance Bank Owned Life Insurance Foreclosed real estate, net Real Estate Acquired Through Foreclosure Other assets Other Assets Total assets Assets Liabilities and Stockholders' Equity Liabilities And Stockholders Equity [Abstract] Deposits: Deposits [Abstract] Noninterest-bearing Noninterest Bearing Deposit Liabilities Interest bearing Interest Bearing Deposit Liabilities Brokered Interest Bearing Deposit Brokered Total deposits Deposits Federal Home Loan Bank of Boston advances Advances From Federal Home Loan Banks Mortgagors' escrow accounts Mortgagors Escrow Accounts Post-employment benefit obligations Postemployment Benefits Liability Noncurrent Other liabilities Other Liabilities Noncurrent Total liabilities Liabilities Commitments and contingencies (Note 16) Commitments And Contingencies Stockholders' Equity: Stockholders Equity [Abstract] Preferred stock, no par value; authorized: 1,000,000 shares; issued: none Preferred Stock Value Common stock, $.01 par value; authorized: 15,000,000 shares; issued and outstanding: 5,576,855 shares at December 31, 2019 and 5,903,793 shares at December 31, 2018 Common Stock Value Additional paid-in capital Additional Paid In Capital Common Stock Retained earnings Retained Earnings Accumulated Deficit ESOP-Unearned compensation Unearned E S O P Shares Accumulated other comprehensive loss, net of tax Accumulated Other Comprehensive Income Loss Net Of Tax Total stockholders' equity Stockholders Equity Total liabilities and stockholders' equity Liabilities And Stockholders Equity Loans, allowance for loan losses Loans And Leases Receivable Allowance Preferred stock, par value Preferred Stock No Par Value Preferred stock, shares authorized Preferred Stock Shares Authorized Preferred stock, shares issued Preferred Stock Shares Issued Common stock, par value Common Stock Par Or Stated Value Per Share Common stock, shares authorized Common Stock Shares Authorized Common stock, shares issued Common Stock Shares Issued Common stock, shares outstanding Common Stock Shares Outstanding Income Statement [Abstract] Interest and dividend income: Interest And Dividend Income Operating [Abstract] Loans Interest And Fee Income Loans And Leases Securities-taxable Dividend Income Securities Operating Taxable Securities-tax exempt Dividend Income Securities Operating Tax Exempt Interest-bearing deposits and certificates of deposit Interest Income And Fees Bankers Acceptances Certificates Of Deposit And Commercial Paper Total interest and dividend income Interest And Dividend Income Operating Interest expense: Interest Expense [Abstract] Deposits Interest Expense Deposits Federal Home Loan Bank of Boston advances Interest Expense Federal Home Loan Bank And Federal Reserve Bank Advances Long Term Total interest expense Interest Expense Net interest income Interest Income Expense Net Provision for loan losses Provision For Loan And Lease Losses Net interest income after provision for loan losses Interest Income Expense After Provision For Loan Loss Non-interest income: Noninterest Income [Abstract] Customer service fees Customer Service Fees Gain on loan origination and sale activities, net Gain On Loan Origination And Sale Activities Net Mortgage servicing fees, net Mortgage Servicing Revenue Expense Gain (loss) on sales/calls of securities and impairment write-down, net Debt And Equity Securities Gain Loss And Impairment Write Down Increase in cash surrender value of life insurance Life Insurance Corporate Or Bank Owned Change In Value Gain on sales of buildings Gain Loss On Disposition Of Assets1 Other Noninterest Income Other Operating Income Total non-interest income Noninterest Income Non-interest expenses: Noninterest Expense [Abstract] Salaries and employee benefits Labor And Related Expense Occupancy and equipment Occupancy Net Data processing Communications And Information Technology Professional fees Professional Fees Marketing Marketing Expense Restructuring charges Restructuring Charges Other Other Noninterest Expense Total non-interest expenses Noninterest Expense Income (loss) before income taxes Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest Income tax expense Income Tax Expense Benefit Net income (loss) Net Income Loss Weighted average common shares outstanding (basic and diluted) Weighted Average Number Of Share Outstanding Basic And Diluted Income (loss) per common share (basic and diluted) Earnings Per Share Basic And Diluted Other comprehensive income (loss), reclassification adjustment from AOCI for sale of securities and impairment write down, before tax. Statement Of Income And Comprehensive Income [Abstract] Statement [Table] Statement [Table] Retirement Plan Type Retirement Plan Type [Axis] Retirement Plan Type Retirement Plan Type [Domain] Supplemental Retirement Plan [Member] Supplemental Employee Retirement Plan Defined Benefit [Member] Statement [Line Items] Statement [Line Items] Net income (loss) Securities available for sale: Other Comprehensive Income Available For Sale Securities Adjustment Net Of Tax Period Increase Decrease [Abstract] Unrealized holding gains (losses) Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Before Tax Reclassification adjustment for net (gains) losses and impairment write-down realized in income Other Comprehensive Income Loss Reclassification Adjustment From A O C I For Sale Of Securities And Impairment Write Down Before Tax Net unrealized gain (loss) Other Comprehensive Income Loss Available For Sale Securities Adjustment Before Tax Related tax effects Other Comprehensive Income Loss Available For Sale Securities Tax Net-of-tax amount Other Comprehensive Income Loss Available For Sale Securities Adjustment Net Of Tax Actuarial losses Other Comprehensive Income Loss Reclassification Adjustment From A O C I Pension And Other Postretirement Benefit Plans For Net Gain Loss Before Tax Prior service credits recognized Other Comprehensive Income Loss Amortization Adjustment From A O C I Pension And Other Postretirement Benefit Plans For Net Prior Service Cost Credit Before Tax Actuarial gains (losses) arising during the year Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Net Unamortized Gain Loss Arising During Period Before Tax Net change in supplemental retirement plan Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Adjustment Before Tax Related tax effects Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Tax Net-of-tax amount Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Adjustment Net Of Tax Total other comprehensive income (loss) Other Comprehensive Income Loss Net Of Tax Comprehensive income (loss) Comprehensive Income Net Of Tax Share redemption for tax with holdings for restricted stock vesting, shares. Share redemption for tax with holdings for restricted stock vesting, value. Employee stock ownership plan ESOP shares committed to be released value. Statement Of Stockholders Equity [Abstract] Common Stock [Member] Statement Equity Components [Axis] Equity Component Equity Component [Domain] Common Stock [Member] Common Stock [Member] Additional Paid-in Capital [Member] Additional Paid In Capital [Member] Retained Earnings [Member] Retained Earnings [Member] Unearned compensation ESOP. Unearned Compensation ESOP [Member] Unearned Compensation E S O P [Member] Accumulated Other Comprehensive Loss [Member] Accumulated Other Comprehensive Income [Member] Beginning balance Beginning balance, shares Shares Outstanding Other comprehensive income (loss) Stock repurchased Stock Repurchased During Period Value Stock repurchased, shares Stock Repurchased During Period Shares Restricted stock awards granted Stock Issued During Period Value Restricted Stock Award Gross Restricted stock awards granted, shares Stock Issued During Period Shares Restricted Stock Award Gross Restricted stock awards forfeited Stock Issued During Period Value Restricted Stock Award Forfeitures Restricted stock awards forfeited, shares Stock Issued During Period Shares Share Based Compensation Forfeited Share redemption for tax withholdings for restricted stock vesting Share Redemption For Tax With Holdings For Restricted Stock Vesting Value Share redemption for tax with holdings for restricted stock vesting, shares Share Redemption For Tax With Holdings For Restricted Stock Vesting Shares Stock-based compensation Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value ESOP shares committed to be released Employee Stock Ownership Plan E S O P Shares Committed To Be Released Value Ending balance Ending balance, shares Statement Of Cash Flows [Abstract] Cash flows from operating activities: Net Cash Provided By Used In Operating Activities [Abstract] Net income (loss) Profit Loss Adjustments to reconcile net income (loss) to net cash used in operating activities: Adjustments To Reconcile Net Income Loss To Cash Provided By Used In Operating Activities [Abstract] Loans originated for sale Payments For Origination Of Mortgage Loans Held For Sale Net gain on sales of mortgage loans Gain Loss On Sale Of Mortgage Loans Proceeds from sales of mortgage loans Proceeds From Sale Of Mortgage Loans Held For Sale Net amortization of securities Accretion Amortization Of Discounts And Premiums Investments Net change in deferred loan costs and fees, and purchase premiums Increase Decrease In Loans Deferred Income (Gain) loss on sales/calls of securities and impairment write-down Debt And Equity Securities Gain Loss Gain on sales of buildings Gain Loss On Sale Of Properties Depreciation and amortization Depreciation Amortization And Accretion Net Impairment write-down on property and equipment Impairment Of Long Lived Assets To Be Disposed Of Impairment write-down on foreclosed real estate Impairment Of Foreclosed Real Estate Stock-based compensation Share Based Compensation ESOP expense Employee Stock Ownership Plan E S O P Compensation Expense Increase in cash surrender value of life insurance Increase Decrease In Life Insurance Liabilities Net increase in mortgage servicing rights Increase Decrease In Mortgage Servicing Rights Other, net Other Operating Activities Cash Flow Statement Net cash used in operating activities Net Cash Provided By Used In Operating Activities Cash flows from investing activities: Net Cash Provided By Used In Investing Activities [Abstract] Redemptions of certificates of deposit Payments For Proceeds From Acquiring Certificate Of Deposit Securities available for sale: Payments For Proceeds From Investments [Abstract] Sales Proceeds From Sale Of Available For Sale Securities Calls/maturities Proceeds From Maturities Prepayments And Calls Of Available For Sale Securities Purchases Payments To Acquire Available For Sale Securities Principal payments on mortgage-backed securities Proceeds From Pay Downs Of Mortgage Backed Securities Available For Sale Loan originations, net of principal repayments Payments For Proceeds From Loans And Leases Proceeds from sale of portfolio loans Proceeds From Sale Of Portfolio Loans And Leases Held For Investment Loan purchases and participations Payments To Acquire Loans Receivable Redemptions (purchases) of Federal Home Loan Bank of Boston stock, net Payments For Proceeds From Federal Home Loan Bank Stock Proceeds from sale of foreclosed real estate Proceeds From Sale Of Foreclosed Assets Proceeds from sales of buildings Proceeds From Sale Of Buildings Purchases of premises and equipment Payments To Acquire Property Plant And Equipment Net cash provided by (used in) investing activities Net Cash Provided By Used In Investing Activities Interest paid on deposits and borrowed funds. Donation of land Transfer of property and equipment to assets held for sale. Cash flows from financing activities: Net Cash Provided By Used In Financing Activities [Abstract] Net increase in non-brokered deposits Net Increase In Non Brokered Deposits Net increase in brokered deposits Net Increase Decrease In Brokered Deposits Net increase (decrease) in short-term Federal Home Loan Bank of Boston borrowings Increase Decrease In Loans To Federal Home Loan Banks Issuance of long-term Federal Home Loan Bank of Boston advances Increase Decrease In Long Term Loans To Federal Home Loan Banks Repayments of long-term Federal Home Loan Bank of Boston advances Repayments Of Federal Home Loan Bank Borrowings Net increase (decrease) in mortgagors' escrow accounts Increase Decrease In Mortgagors Escrow Accounts Net Repurchases of common stock Payments For Repurchase Of Common Stock Stock repurchase payable Stock Repurchase Payable Net cash provided by financing activities Net Cash Provided By Used In Financing Activities Net change in cash and cash equivalents Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect Cash and cash equivalents at beginning of year Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Including Disposal Group And Discontinued Operations Cash and cash equivalents at end of year Supplemental cash flow information: Supplemental Cash Flow Information [Abstract] Interest paid on deposits and borrowed funds Interest Paid On Deposits And Borrowed Funds Income taxes paid (refunded), net Income Taxes Paid Net Non-cash items: Cash Flow Noncash Investing And Financing Activities Disclosure [Abstract] Donation of land, net Donation Of Land Transfer of portfolio loans to held for sale Transfer Of Loans Held For Sale To Portfolio Loans1 Transfer of property and equipment to assets held for sale Transfer Of Property And Equipment To Assets Held For Sale Organization Consolidation And Presentation Of Financial Statements [Abstract] Basis of Presentation and Summary of Significant Accounting Policies Basis Of Presentation And Significant Accounting Policies [Text Block] Investments Debt And Equity Securities [Abstract] Securities Available for Sale Investments In Debt And Marketable Equity Securities And Certain Trading Assets Disclosure [Text Block] Receivables [Abstract] Loans Loans Notes Trade And Other Receivables Disclosure [Text Block] Transfers And Servicing [Abstract] Loan Servicing Transfers And Servicing Of Financial Assets [Text Block] Property Plant And Equipment [Abstract] Premises and Equipment Property Plant And Equipment Disclosure [Text Block] Deposits Deposit Liabilities Disclosures [Text Block] Debt Disclosure [Abstract] Borrowings Debt Disclosure [Text Block] Income Tax Disclosure [Abstract] Income Taxes Income Tax Disclosure [Text Block] Derivative Instruments And Hedging Activities Disclosure [Abstract] On-Balance Sheet Derivative Instruments and Hedging Activities Derivative Instruments And Hedging Activities Disclosure [Text Block] Banking And Thrift [Abstract] Minimum Regulatory Capital Requirements Regulatory Capital Requirements Under Banking Regulations [Text Block] Compensation And Retirement Disclosure [Abstract] Post-retirement Plans Pension And Other Postretirement Benefits Disclosure [Text Block] Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] Stock-Based Compensation Shareholders Equity And Share Based Payments [Text Block] Employee Stock Ownership Plan Disclosure Of Compensation Related Costs Share Based Payments [Text Block] Equity [Abstract] Share Repurchase Program Stockholders Equity Note Disclosure [Text Block] Earnings Per Share [Abstract] Earnings (Loss) Per Share Earnings Per Share [Text Block] Commitments And Contingencies Disclosure [Abstract] Commitments and Contingencies Commitments And Contingencies Disclosure [Text Block] Fair Value Disclosures [Abstract] Fair Value of Assets and Liabilities Fair Value Disclosures [Text Block] Segment Reporting [Abstract] Segment Information Segment Reporting Disclosure [Text Block] Other non interest expenses. Other Income And Expenses [Abstract] Other Non-Interest Expenses Other Non Interest Expenses Disclosure [Text Block] Restructuring And Related Activities [Abstract] Restructuring Charge Restructuring And Related Activities Disclosure [Text Block] Condensed Financial Information Of Parent Company Only Disclosure [Abstract] Parent Company Condensed Financial Statements Condensed Financial Information Of Parent Company Only Disclosure [Text Block] Quarterly Financial Information Disclosure [Abstract] Selected Quarterly Financial Data (Unaudited) Quarterly Financial Information [Text Block] Subsequent Events [Abstract] Subsequent Event Subsequent Events [Text Block] Accounting Policies [Abstract] Nature of Operations Nature Of Operations Basis of Presentation Basis Of Accounting Policy Policy [Text Block] Use of Estimates Use Of Estimates Cash and Cash Equivalents Cash And Cash Equivalents Policy [Text Block] Certificates of deposit policy policy text block. Certificates of Deposit Certificates Of Deposit Policy Policy [Text Block] Fair Value Hierarchy Fair Value Measurement Policy Policy [Text Block] Securities Marketable Securities Policy Investment in federal home loan bank stock. Federal Home Loan Bank of Boston Stock Investment In Federal Home Loan Bank Stock Policy [Text Block] Loans Held for Sale and related derivatives Finance Loan And Lease Receivables Held For Sale Policy Loans Finance Loans And Leases Receivable Policy Allowance for Loan Losses Loans And Leases Receivable Allowance For Loan Losses Policy Bank owned life insurance policy text block. Bank-owned Life Insurance Bank Owned Life Insurance Policy [Text Block] Mortgage Servicing Rights Loans And Leases Receivable Mortgage Banking Activities Policy Premises and Equipment Property Plant And Equipment Policy [Text Block] Transfers of Financial Assets Transfers And Servicing Of Financial Assets Policy [Text Block] Foreclosed real estate. Foreclosed Real Estate Foreclosed Real Estate Policy [Text Block] Supplemental Retirement Plans Pension And Other Postretirement Plans Pensions Policy Employee Stock Ownership Plan Employee Stock Ownership Plan E S O P Policy Stock-Based Compensation Share Based Compensation Option And Incentive Plans Policy Advertising Cost [Policy Text Block] Advertising Costs Policy [Text Block] Income Taxes Income Tax Policy [Text Block] Comprehensive Income (Loss) Comprehensive Income Policy Policy [Text Block] Segment Reporting Segment Reporting Policy Policy [Text Block] Earnings (Loss) Per Share Earnings Per Share Policy [Text Block] Business Combinations Business Combinations Policy Recent Accounting Pronouncements New Accounting Pronouncements Policy Policy [Text Block] Reclassifications Prior Period Reclassification Adjustment Description Components of Accumulated Other Comprehensive Loss (AOCI) and Related Tax Effects Schedule Of Comprehensive Income Loss Table [Text Block] Schedule of Amortized Cost and Fair Value of Securities Available For Sale Securities [Text Block] Investments Classified by Contractual Maturity Date Investments Classified By Contractual Maturity Date Table [Text Block] Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value Available For Sale Securities Continuous Unrealized Loss Position Fair Value Table [Text Block] Summary of Loan Portfolio Schedule Of Accounts Notes Loans And Financing Receivable [Text Block] Summary of Activity in the Allowance for Loan Losses by Loan Category Schedule Of Credit Losses Related To Financing Receivables Noncurrent Table [Text Block] Summary of Additional Information Pertaining to the Allowance for Loan Losses Allowance For Credit Losses On Financing Receivables Table [Text Block] Schedule of Past Due and Non-Accrual Loans by Loan Category Past Due Financing Receivables Table [Text Block] Summary of Impaired Loans and Information Related to Average Balances of Impaired Loans and Interest Income Recognized Impaired Financing Receivables Table [Text Block] Summary of Company's Loans by Risk Rating Financing Receivable Credit Quality Indicators Table [Text Block] Summary of Activity Relating to Mortgage Servicing Rights Schedule Of Servicing Assets At Fair Value [Text Block] Summary of Cost and Accumulated Depreciation and Amortization of Premises and Equipment Property Plant And Equipment [Text Block] Summary of deposit balances by type. Summary of Deposit Balances by Type Summary Of Deposit Balances By Type Table [Text Block] Summary of term certificates by maturity. Summary of Term Certificates, Including Brokered Deposits, by Maturity Summary Of Term Certificates By Maturity Table [Text Block] Summary of Borrowings from FHLBB Federal Home Loan Bank Advances Table [Text Block] Schedule of federal home loan bank, advances, outstanding balances. Summary of Outstanding Balances of Short Term Borrowings from FHLBB Schedule Of Federal Home Loan Bank Advances Outstanding Balances Table [Text Block] Allocation of Federal and State Income Taxes Between Current and Deferred Portions Schedule Of Components Of Income Tax Expense Benefit Table [Text Block] Summary of Differences Between Statutory Federal Income Tax Expense (Benefit) and Actual Tax Expense Schedule Of Effective Income Tax Rate Reconciliation Table [Text Block] Schedule of components of net deferred tax assets. Summary of Components of Net Deferred Tax Asset Schedule Of Components Of Net Deferred Tax Assets Table [Text Block] Summary of Deferred Tax Assets (Liabilities) Schedule Of Deferred Tax Assets And Liabilities Table [Text Block] Summary of Actual and Minimum Capital Amounts and Ratios Exclusive of Capital Conservation Buffer Schedule Of Compliance With Regulatory Capital Requirements Under Banking Regulations [Text Block] Summary of Information Pertaining to Activity in Plan Changes In Projected Benefit Obligations Fair Value Of Plan Assets And Funded Status Of Plan Table [Text Block] Summary of Assumptions Used to Determine Benefit Obligation Schedule Of Assumptions Used Table [Text Block] Summary of Net Periodic Benefit Cost Included in Salaries and Employee Benefits Expense Schedule Of Costs Of Retirement Plans Table [Text Block] Summary of Estimated Future Benefit Payments Expected Future Services Schedule Of Expected Benefit Payments Table [Text Block] Summary of Grants of Options to Purchase Shares of Common Stock Schedule Of Share Based Payment Award Stock Options Valuation Assumptions Table [Text Block] Summary of Activity and Certain Other Information Related to Stock Options Grants Under Equity Plan Schedule Of Share Based Compensation Stock Options Activity Table [Text Block] Summary of Activity and Certain Other Information Related to Restricted Stock Awards Under Equity Plan Schedule Of Sharebased Compensation Restricted Stock And Restricted Stock Units Activity Table [Text Block] Schedule of Calculation of Average Number of Shares Outstanding Used to Calculate Basic and Diluted Earnings (Loss) Per Share Schedule Of Weighted Average Number Of Shares Table [Text Block] Schedule of Future Minimum Rent Commitments Lessee Operating Lease Liability Maturity Table [Text Block] Summary of Financial Instruments Outstanding Contract Amounts Represent Credit Risk Schedule Of Fair Value Off Balance Sheet Risks [Text Block] Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis Schedule Of Fair Value Assets And Liabilities Measured On Recurring Basis Table [Text Block] Schedule of Assets Measured at Fair Value on a Non-Recurring Basis Fair Value Assets Measured On Nonrecurring Basis [Text Block] Summary of Carrying Values, Estimated Fair Values and Placement in Fair Value Hierarchy of Company's Financial Instruments Fair Value By Balance Sheet Grouping [Text Block] Summary of Segment Information Schedule Of Segment Reporting Information By Segment [Text Block] Schedule of Company's Total Interest and Non-Interest Income Included in Other Non-Interest Expenses Schedule Of Other Operating Cost And Expense By Component [Text Block] Balance Sheets Schedule Of Condensed Balance Sheet Table [Text Block] Statements of Operations Schedule Of Condensed Income Statement Table [Text Block] Statements of Cash Flows Schedule Of Condensed Cash Flow Statement Table [Text Block] Summary of Selected Quarterly Financial Data Schedule Of Quarterly Financial Information Table [Text Block] Summary of significant accounting policies line items. Summary of significant accounting policies table. Summary Of Significant Accounting Policies [Table] Summary Of Significant Accounting Policies [Table] Statistical Measurement Range [Axis] Statistical Measurement Range [Member] Maximum [Member] Maximum [Member] Minimum [Member] Minimum [Member] Receivable Type Accounts Notes Loans And Financing Receivable By Receivable Type [Axis] Receivable Receivable Type [Domain] Residential one-to-four family real estate portfolio segment. Residential One-to-Four Family Real Estate Portfolio Segment [Member] Residential One To Four Family Real Estate Portfolio Segment [Member] Second mortgages and home equity lines of credit property segment. Second Mortgages and Home Equity Lines of Credit Property Segment [Member] Second Mortgages And Home Equity Lines Of Credit Property Segment [Member] Consumer loans or second mortgages and HELOCs. Consumer Loans or Second Mortgages and HELOCs [Member] Consumer Loans Or Second Mortgages And H E L O Cs [Member] Summary Of Significant Accounting Policies [Line Items] Summary Of Significant Accounting Policies [Line Items] FDIC insurance coverage limit Time Deposits100000 Or More Cash and cash equivalents maturity period. Cash equivalents maturity period Cash And Cash Equivalent Maturity Period Certificates of deposit maturity period. Certificates of deposit maturity period Certificates Of Deposit Maturity Period Time period to capture relevant loan loss data. Time period to capture relevant loan loss data Time Period To Capture Relevant Loan Loss Data Loan to value ratio. Loan-to-value ratio Loan To Value Ratio Impairment disclosure requirement. Impairment disclosure requirement Impairment Disclosure Requirement Obligation to repurchase mortgage loans prescribed time period. Obligation to repurchase mortgage loans prescribed time period Obligation To Repurchase Mortgage Loans Prescribed Time Period Percentage of tax benefit likely to be realized on examination. Description of tax benefit Income Tax Examination Likelihood Of Unfavorable Settlement Percentage of tax benefit that likely of being realized on examination with taxing authority Percentage Of Tax Benefit Likely To Be Realized On Examination Uncertain tax positions Unrecognized Tax Benefits Interest or penalties Income Tax Examination Penalties And Interest Expense Accumulated Other Comprehensive Income Loss [Table] Accumulated Other Comprehensive Income Loss [Table] Accumulated Net Investment Gain (Loss) Attributable to Parent Accumulated Net Unrealized Investment Gain Loss [Member] Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent [Member] Accumulated Defined Benefit Plans Adjustment Net Unamortized Gain Loss [Member] Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent [Member] Accumulated Defined Benefit Plans Adjustment Net Prior Service Cost Credit [Member] Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] Accumulated Defined Benefit Plans Adjustment [Member] Accumulated Other Comprehensive Income Loss [Line Items] Accumulated Other Comprehensive Income Loss [Line Items] Net unrealized gain (loss) Other Comprehensive Income Loss Before Tax Other comprehensive Loss, Tax effect Other Comprehensive Income Loss Tax Accumulated other comprehensive loss Prior service credits Defined Benefit Plan Expected Amortization Of Prior Service Cost Credit Next Fiscal Year Net actuarial losses Defined Benefit Plan Expected Amortization Of Gain Loss Next Fiscal Year Number of business segments Number Of Reportable Segments Anti-dilutive securities excluded from computation of earnings (loss) per share Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount Schedule Of Available For Sale Securities [Table] Schedule Of Available For Sale Securities [Table] Financial Instrument Financial Instrument [Axis] Financial Instruments Transfers And Servicing Of Financial Instruments Types Of Financial Instruments [Domain] Debt Securities [Member] Debt Securities [Member] Equity Securities [Member] Equity Securities [Member] Investment Type Investment Type [Axis] Investments Investment Type Categorization [Member] US Government-sponsored Enterprises [Member] U S Government Sponsored Enterprises Debt Securities [Member] Corporate [Member] Corporate Debt Securities [Member] Municipal [Member] Municipal Bonds [Member] Residential Mortgage-backed Securities, US Government Sponsored Enterprises [Member] Mortgage Backed Securities Issued By U S Government Sponsored Enterprises [Member] Commercial mortgage backed securities issued by US government sponsored enterprises. Commercial Mortgage-backed Securities, US Government Sponsored Enterprises [Member] Commercial Mortgage Backed Securities Issued By U S Government Sponsored Enterprises [Member] Commercial Mortgage Backed Securities, U.S. Government-guaranteed [Member] Commercial Mortgage Backed Securities [Member] Collateralized mortgage obligations issued by US government sponsored enterprises. Collateralized Mortgage Obligations, US Government Sponsored Enterprises [Member] Collateralized Mortgage Obligations Issued By U S Government Sponsored Enterprises [Member] Collateralized mortgage obligations issued by US government guaranteed. Collateralized Mortgage Obligations, US Government Guaranteed [Member] Collateralized Mortgage Obligations Issued By U S Government Guaranteed [Member] Schedule of Available-for-sale Securities [Line Items] Schedule Of Available For Sale Securities [Line Items] Amortized Cost Available For Sale Securities Amortized Cost Gross Unrealized Gains Available For Sale Securities Accumulated Gross Unrealized Gain Before Tax Gross Unrealized Losses Available For Sale Securities Accumulated Gross Unrealized Loss Before Tax Fair Value Available-for-sale Securities, Debt Maturities, Amortized Cost Available For Sale Securities Debt Maturities Amortized Cost [Abstract] Within 1 year Available For Sale Securities Debt Maturities Within One Year Amortized Cost After 1 year through 5 years Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost After 5 years through 10 years Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis, Total Available For Sale Securities Debt Maturities Single Maturity Date Amortized Cost Basis Mortgage-backed securities and collateralized mortgage obligations Available For Sale Securities Debt Maturities Without Single Maturity Date Amortized Cost Available-for-sale Debt Securities, Amortized Cost Basis, Total Available For Sale Debt Securities Amortized Cost Basis Available-for-sale Securities, Debt Maturities, Fair Value Available For Sale Securities Debt Maturities Fair Value [Abstract] Within 1 year Available For Sale Securities Debt Maturities Within One Year Fair Value After 1 year through 5 years Available For Sale Securities Debt Maturities After One Through Five Years Fair Value After 5 years through 10 years Available For Sale Securities Debt Maturities After Five Through Ten Years Fair Value Available-for-sale Securities, Debt Maturities, Single Maturity Date, Fair Value Total Available For Sale Securities Debt Maturities Single Maturity Date Mortgage-backed securities and collateralized mortgage obligations Available For Sale Securities Debt Maturities Without Single Maturity Date Fair Value Available-for-sale Securities, Debt Securities, Fair Value Total Available For Sale Securities Debt Securities Prepayment of Commercial Mortgage-backed Security [Member] OTTI Write-down of Mutual Fund [Member] Mutual Fund [Member] Fair value of investment securities pledged as collateral Security Owned And Pledged As Collateral Fair Value Investment securities on equity Investment Owned At Fair Value Less Than Twelve Months, Gross Unrealized Losses Available For Sale Securities Continuous Unrealized Loss Position Less Than12 Months Accumulated Loss Less Than Twelve Months, Fair Value Available For Sale Securities Continuous Unrealized Loss Position Less Than Twelve Months Fair Value Over Twelve Months, Gross Unrealized Losses Available For Sale Securities Continuous Unrealized Loss Position12 Months Or Longer Accumulated Loss Over Twelve Months, Fair Value Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or Longer Fair Value Number of debt securities with unrealized losses. Unrealized losses debt securities aggregate depreciation percentage. Number of debt securities with unrealized losses Number Of Debt Securities With Unrealized Losses Unrealized losses debt securities aggregate depreciation percentage Unrealized Losses Debt Securities Aggregate Depreciation Percentage Proceeds from sales of available-for-sale securities Available-for-sale securities, gross realized gains Available For Sale Securities Gross Realized Gains Available-for-sale securities, gross realized losses Available For Sale Securities Gross Realized Losses Accounts Notes Loans And Financing Receivable [Abstract] Accounts Notes Loans And Financing Receivable [Abstract] Schedule Of Accounts Notes Loans And Financing Receivable [Table] Schedule Of Accounts Notes Loans And Financing Receivable [Table] Class of Financing Receivable Financing Receivable Recorded Investment By Class Of Financing Receivable [Axis] Class of Financing Receivable Financing Receivable Recorded Investment Class Of Financing Receivable [Domain] 1-4 family. One-to-Four Family [Member] One To Four Family Loans [Member] Home equity loans and lines of credit. Home Equity Loans and Lines of Credit [Member] Home Equity Loans And Lines Of Credit [Member] Construction [Member] Construction Loans [Member] Financing Receivable Portfolio Segment Financing Receivable Portfolio Segment [Axis] Financing Receivable Portfolio Segment Financing Receivable Portfolio Segment [Domain] Residential Real Estate [Member] Residential Portfolio Segment [Member] Commercial Real Estate Loans [Member] Commercial Real Estate Portfolio Segment [Member] Commercial and industrial. Commercial and Industrial [Member] Commercial And Industrial Portfolio Segment [Member] Consumer [Member] Consumer Portfolio Segment [Member] Industry Sector Equity Securities By Industry [Axis] Industry Sector Equity Securities Industry [Member] Real Estate Sector [Member] Real Estate Sector [Member] Accounts, Notes, Loans and Financing Receivable [Line Items] Accounts Notes And Loans Receivable [Line Items] Total loans Loans And Leases Receivable Gross Carrying Amount Allowance for loan losses Net deferred loan costs and fees, and purchase premiums Loans And Leases Receivable Net Of Deferred Income Loans, net Non accrual loans. Non Accrual Loans [Member] Non Accrual Loans [Member] Financial Asset, Period Past Due Financing Receivables Period Past Due [Axis] Financial Asset, Period Past Due Financing Receivables Period Past Due [Domain] 30 - 59 Days Past Due [Member] Financing Receivables30 To59 Days Past Due [Member] Consumer Loans [Member] Consumer Loan [Member] Residential Mortgage [Member] Residential Mortgage [Member] Internal Credit Assessment Internal Credit Assessment [Axis] Internal Credit Assessment Internal Credit Assessment [Domain] Doubtful [Member] Doubtful [Member] Substandard [Member] Substandard [Member] Special Mention [Member] Special Mention [Member] Servicing loans for participants Servicing Asset Schedule Of Financing Receivable Allowance For Credit Losses [Table] Schedule Of Financing Receivable Allowance For Credit Losses [Table] Second mortgages and HELOC. Second Mortgages and HELOC [Member] Second Mortgages And Home Equity Loans And Lines Of Credit [Member] Financing Receivable, Allowance for Credit Losses [Line Items] Financing Receivable Allowance For Credit Losses [Line Items] Beginning balance Financing Receivable Allowance For Credit Losses Provision (credit) for loan losses Loans charged-off Financing Receivable Allowance For Credit Losses Write Offs Recoveries Financing Receivable Allowance For Credit Losses Recovery Ending balance Allowance for impaired loans Financing Receivable Allowance For Credit Losses Individually Evaluated For Impairment1 Allowance for non-impaired loans Financing Receivable Allowance For Credit Losses Collectively Evaluated For Impairment Total allowance for loan losses Impaired loans Financing Receivable Individually Evaluated For Impairment Non-impaired loans Financing Receivable Collectively Evaluated For Impairment Total loans Schedule Of Financing Receivables Past Due [Table] Schedule Of Financing Receivables Past Due [Table] 60 - 89 Days Past Due [Member] Financing Receivables60 To89 Days Past Due [Member] 90 Days or More Past Due [Member] Financing Receivables Equal To Greater Than90 Days Past Due [Member] Financing Receivable, Recorded Investment, Past Due [Line Items] Financing Receivable Recorded Investment Past Due [Line Items] Total Past Due Financing Receivable Recorded Investment Past Due Non-accrual Loans Financing Receivable Recorded Investment Nonaccrual Status Impaired financing receivable with no related allowance. Schedule Of Impaired Financing Receivable [Table] Schedule Of Impaired Financing Receivable [Table] Financing Receivable, Impaired [Line Items] Financing Receivable Impaired [Line Items] Recorded Investment without a valuation allowance Impaired Financing Receivable With No Related Allowance Recorded Investment Unpaid Principal Balance without a valuation allowance Impaired Financing Receivable With No Related Allowance Unpaid Principal Balance Related Allowance without a valuation allowance Impaired Financing Receivable With No Related Allowance Recorded Investment with a valuation allowance Impaired Financing Receivable With Related Allowance Recorded Investment Unpaid Principal Balance with a valuation allowance Impaired Financing Receivable With Related Allowance Unpaid Principal Balance Related Allowance, Total impaired loans Impaired Financing Receivable Related Allowance Recorded Investment, Total impaired loans Impaired Financing Receivable Recorded Investment Unpaid Principal Balance, Total impaired loans Impaired Financing Receivable Unpaid Principal Balance Loan And Lease Receivables Impaired [Abstract] Average Recorded Investment Impaired Financing Receivable Average Recorded Investment Interest Income Recognized Impaired Financing Receivable Interest Income Accrual Method Cash Basis Interest Recognized Impaired Financing Receivable Interest Income Cash Basis Method Impaired loans, additional funds committed Loans And Leases Receivable Impaired Commitment To Lend Troubled debt restructuring interest rate concession period. Troubled debt restructuring, interest rate concession period Troubled Debt Restructuring Interest Rate Concession Period Loans subject to troubled debt restructurings Financing Receivable Modifications Number Of Contracts2 Loans subject to troubled debt restructurings, amount Financing Receivable Modifications Recorded Investment Troubled debt restructuring amount Financing Receivable Modifications Post Modification Recorded Investment2 Allowance for credit losses change in method of calculating troubled debt restructuring. Allowances related to troubled debt restructurings Allowance For Credit Losses Change In Method Of Calculating Troubled Debt Restructuring Troubled debt restructurings defaulted over 30 days past due Financing Receivable Modifications Subsequent Default Recorded Investment1 Pass [Member] Pass [Member] Minimum past due days for loan rating. Minimum past due days for loan rating Minimum Past Due Days For Loan Rating Unpaid principal balance of residential mortgage loans serviced for others. Servicing Assets At Amortized Value [Table] Servicing Assets At Amortized Value [Table] Measurement Input Type Measurement Input Type [Axis] Measurement Input Type Measurement Input Type [Domain] Measurement Input, Discount Rate [Member] Measurement Input Discount Rate [Member] Measurement Input, Constant Prepayment Rate [Member] Measurement Input Constant Prepayment Rate [Member] Servicing Assets At Amortized Value [Line Items] Servicing Assets At Amortized Value [Line Items] Unpaid principal balances of residential mortgage loans serviced for others Unpaid Principal Balances Of Residential Mortgage Loans Serviced For Others Mortgage servicing rights, net. Mortgage servicing rights: Servicing Asset At Amortized Value Balance Roll Forward Balance at beginning of year Additions through originations Servicing Asset At Amortized Value Additions Amortization Servicing Asset At Amortized Value Amortization1 Balance at end of year Servicing Asset At Amortized Value Valuation allowance: Valuation Allowance For Impairment Of Recognized Servicing Assets Roll Forward Balance at beginning of year Valuation Allowance For Impairment Of Recognized Servicing Assets Balance Provision (credit) Valuation Allowance For Impairment Of Recognized Servicing Assets Provisions Recoveries Balance at end of year Mortgage servicing rights, amortized cost Mortgage Servicing Rights Net Mortgage servicing rights, fair value Servicing Asset At Fair Value Amount Discount rate Servicing Asset Measurement Input Increase (reduced) the valuation allowance of mortgage servicing rights Valuation Allowance For Impairment Of Recognized Servicing Assets Period Increase Decrease Contractually specified servicing fees, net Contractually Specified Servicing Fees Amount Schedule Of Property Plant And Equipment [Table] Schedule Of Property Plant And Equipment [Table] Property, Plant and Equipment, Type Property Plant And Equipment By Type [Axis] Property, Plant and Equipment, Type Property Plant And Equipment Type [Domain] Buildings and Improvements [Member] Building And Building Improvements [Member] Leasehold Improvements [Member] Leasehold Improvements [Member] Furniture and Equipment [Member] Furniture And Fixtures [Member] Property Plant And Equipment [Line Items] Property Plant And Equipment [Line Items] Land and improvements Land And Land Improvements Buildings and improvements Buildings And Improvements Gross Leasehold improvements Leasehold Improvements Gross Furniture and equipment Furniture And Fixtures Gross Construction-in-progress Construction In Progress Gross Property plant and equipment, gross Property Plant And Equipment Gross Less accumulated depreciation and amortization Accumulated Depreciation Depletion And Amortization Property Plant And Equipment Property plant and equipment, net Estimated Useful Life Property Plant And Equipment Useful Life Restructuring Plan Restructuring Plan [Axis] Restructuring Plan Restructuring Plan [Domain] Consolidation of mortgage banking operations. Consolidation of Mortgage Banking Operations [Member] Consolidation Of Mortgage Banking Operations [Member] Geographical Statement Geographical [Axis] Geographical Segment Geographical [Domain] Andover. Andover [Member] Andover [Member] Income Statement Location Income Statement Location [Axis] Income Statement Location Income Statement Location [Domain] Restructuring Charges [Member] Restructuring Charges [Member] Long Lived Assets Held-for-sale by Asset Type Long Lived Assets Held For Sale By Asset Type [Axis] Long Lived Assets Held-for-sale, Name Long Lived Assets Held For Sale Name [Domain] Branch office one. Branch Office, Other [Member] Branch Office One [Member] Branch office. Branch Office [Member] Branch Office [Member] Boston. Boston [Member] Boston [Member] Loss Contingency Nature Loss Contingencies By Nature Of Contingency [Axis] Loss Contingency, Nature Loss Contingency Nature [Domain] Insurance Recovery [Member] Insurance Claims [Member] Depreciation and amortization expense Depreciation And Amortization Leasehold improvements funded by landlord. Leasehold improvements allowance amortization period. Tenant improvement allowance amortization period. Leasehold improvements funded by landlord Leasehold Improvements Funded By Landlord Leasehold improvements allowance amortized period Leasehold Improvements Allowance Amortization Period Tenant improvement allowance amortized period Tenant Improvement Allowance Amortization Period Percentage of land occupied under sublease agreement. Termination of lease. Percentage of land occupied under sublease agreement Percentage Of Land Occupied Under Sublease Agreement Termination of lease Termination Of Lease Fair value, cease use liability. Fair value, Cease use liability Fair Value Cease Use Liability Impairment write-down of assets held for sale Number of branches sold. Number of branch office replaced. Number of buildings reclassified to assets held for sale. Number of branches sold Number Of Branches Sold Number of branch office replaced Number Of Branch Office Replaced Number of buildings reclassified to assets held for sale Number Of Buildings Reclassified To Assets Held For Sale Gain (loss) on sale of building Consideration received Disposal Group Including Discontinued Operation Consideration Insurance recovery for personal property Loss Contingency Receivable Proceeds Brokered deposits. Demand deposits Demand Deposit Accounts NOW accounts Deposits Negotiable Order Of Withdrawal N O W Money market deposits Deposits Money Market Deposits Regular and other savings accounts Deposits Savings Deposits Brokered deposits Brokered Deposits Total non-certificate accounts Deposits Money Market Deposits And Negotiable Order Of Withdrawal N O W Term certificates less than $250,000 Time Deposits Less Than250000 Term certificates of $250,000 or more Time Deposits250000 Or More Term certificates - brokered Time Deposits Brokered Total certificate accounts Time Deposits Exercise of right to call option, term certificates. Subsequent Event [Table] Subsequent Event [Table] Option Indexed to Issuer's Equity, Type Option Indexed To Issuers Equity Type [Axis] Option Indexed to Issuer's Equity, Type Option Indexed To Issuers Equity Type [Domain] Call Option [Member] Call Option [Member] Subsequent Event Type Subsequent Event Type [Axis] Subsequent Event Type Subsequent Event Type [Domain] Subsequent Event [Member] Subsequent Event [Member] Subsequent Event [Line Items] Subsequent Event [Line Items] Weighted average rate, term certificate Weighted Average Rate Domestic Deposit Time Deposits Exercise of right to call option, term certificates Exercise Of Right To Call Option Term Certificates Amount Maturities Of Time Deposits [Abstract] 2019 Time Deposit Maturities Year One 2020 Time Deposit Maturities Year Two 2021 Time Deposit Maturities Year Three 2022 Time Deposit Maturities Year Four 2023 Time Deposit Maturities Year Five 2024 Time Deposit Maturities After Year Five Weighted Average Rate Weighted Average Rate Of Time Deposits [Abstract] 2019 Time Deposits Weighted Average Interest Rate Maturities Year One 2020 Time Deposits Weighted Average Interest Rate Maturities Year Two 2021 Time Deposits Weighted Average Interest Rate Maturities Year Three 2022 Time Deposits Weighted Average Interest Rate Maturities Year Four 2023 Time Deposits Weighted Average Interest Rate Maturities Year Five 2024 Time Deposits Weighted Average Interest Rate Maturities Greater Than Five Years Total Fixed-rate advances maturing: Federal Home Loan Bank Advances Maturities Summary [Abstract] 2019 Federal Home Loan Bank Advances Maturities Summary Due Within One Year Of Balance Sheet Date 2020 Federal Home Loan Bank Advances Maturities Summary Due From One To Two Years Of Balance Sheet Date 2021 Federal Home Loan Bank Advances Maturities Summary Due From Two To Three Years Of Balance Sheet Date 2022 Federal Home Loan Bank Advances Maturities Summary Due From Three To Four Years Of Balance Sheet Date Advances from FHLBB, Amount Weighted Average Rate Federal Home Loan Bank Advances Maturities Summary Average Interest Rate Of Amounts Due [Abstract] 2019 Federal Home Loan Bank Advances Maturities Summary Average Interest Rate Of Amounts Due Within One Year Of Balance Sheet Date 2020 Federal Home Loan Bank Advances Maturities Summary Average Interest Rate One To Two Years From Balance Sheet Date 2021 Federal Home Loan Bank Advances Maturities Summary Average Interest Rate Two To Three Years From Balance Sheet Date 2022 Federal Home Loan Bank Advances Maturities Summary Average Interest Rate Three To Four Years From Balance Sheet Date Advances from FHLBB, Weighted Average Rate Federal Home Loan Bank Advances Branch Of F H L B Bank Interest Rate Collateral percentage of financial instrument. Debt Instrument [Table] Debt Instrument [Table] Federal Home Loan Bank Branch Federal Home Loan Bank Advances Branch Of F H L B Bank [Axis] Federal Home Loan Bank Branch Federal Home Loan Bank Advances Branch Of F H L B Bank State [Domain] FHLBB [Member] Federal Home Loan Bank Of Boston [Member] Federal home loan bank borrowings with interest rate adjusts daily. FHLBB With Interest Rate Adjusts Daily [Member] Federal Home Loan Bank Borrowings With Interest Rate Adjusts Daily [Member] Federal Reserve Bank of Boston. Federal Reserve Bank of Boston [Member] Federal Reserve Bank Of Boston [Member] Lender Name Line Of Credit Facility [Axis] Line of Credit Facility, Lender Line Of Credit Facility Lender [Domain] Correspondent bank. Correspondent Bank [Member] Correspondent Bank [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] Percentage on carrying value of first mortgage loans pledged as collateral Collateral Percentage Of Financial Instrument Available borrowing capacity Federal Home Loan Bank Advances General Debt Obligations Disclosures Amount Of Available Unused Funds Overnight advances from Federal Home Loan Banks. Overnight advances of FHLBB borrowings Overnight Advances From Federal Home Loan Banks Federal home loan bank advances average daily balance. Federal Home Loan Bank Advances Activity For Year [Abstract] Average daily balance Federal Home Loan Bank Advances Average Daily Balance Maximum outstanding at any month end Federal Home Loan Bank Advances Activity For Year Maximum Outstanding At Any Month End Line of credit facility asset restrictions percentage. Percentage on total assets for borrowings under line of credit Line Of Credit Facility Asset Restrictions Percentage Federal Home Loan Bank advances Current tax expense: Current Income Tax Expense Benefit Continuing Operations [Abstract] Federal Current Federal Tax Expense Benefit State Current State And Local Tax Expense Benefit Total current tax expense Current Income Tax Expense Benefit Deferred tax expense (benefit): Deferred Income Tax Expense Benefit Continuing Operations [Abstract] Federal Deferred Federal Income Tax Expense Benefit State Deferred State And Local Income Tax Expense Benefit Total deferred federal and state tax expense (benefit) Deferred Federal State And Local Tax Expense Benefit Change in valuation allowance Valuation Allowance Deferred Tax Asset Change In Amount Total deferred tax expense (benefit) Deferred Income Tax Expense Benefit Total tax expense Income tax reconciliation increase decrease. Income Tax Expense Benefit Continuing Operations Income Tax Reconciliation [Abstract] Statutory federal tax rate of 21% Income Tax Reconciliation Income Tax Expense Benefit At Federal Statutory Income Tax Rate Increase (decrease) resulting from: Income Tax Reconciliation Increase Decrease [Abstract] State taxes, net of federal tax effect Income Tax Reconciliation State And Local Income Taxes Bank-owned life insurance Income Tax Reconciliation Bank Owned Life Insurance Tax-exempt income Income Tax Reconciliation Tax Exempt Income Change in valuation allowance Income Tax Reconciliation Change In Deferred Tax Assets Valuation Allowance Other, net Income Tax Reconciliation Other Adjustments Statutory federal tax rate Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate Tax cuts and jobs act of 2017 alternative minimum tax refundable period. Income tax. Income tax table. Income Tax [Table] Income Tax [Table] Balance Sheet Location Balance Sheet Location [Axis] Balance Sheet Location Balance Sheet Location [Domain] Other Assets [Member] Other Assets [Member] Tax Credit Carryforward Tax Credit Carryforward [Axis] Tax Credit Carryforward, Name Tax Credit Carryforward Name [Domain] Operating loss carryforward expiration date one. Operating Loss Carryforwards Expires on December 31, 2033 [Member] Operating Loss Carryforward Expiration Date One [Member] Operating loss carryforward expiration date two. Operating Loss Carryforwards Expires on December 31, 2034 [Member] Operating Loss Carryforward Expiration Date Two [Member] Operating loss carryforward expiration date three. Operating Loss Carryforward Expires on December 31, 2035 [Member] Operating Loss Carryforward Expiration Date Three [Member] Operating loss carryforward expiration date four. Operating Loss Carryforward Expires on December 31, 2036 [Member] Operating Loss Carryforward Expiration Date Four [Member] Operating loss carryforward expiration date five. Operating Loss Carryforward Expires on December 31, 2037 [Member] Operating Loss Carryforward Expiration Date Five [Member] Operating loss carryforward no expiration. Operating Loss Carryforward No Expiration [Member] Operating Loss Carryforward No Expiration [Member] Income Tax Authority Income Tax Authority [Axis] Income Tax Authority Income Tax Authority [Domain] Federal [Member] Domestic Country [Member] Deferred tax assets charitable contribution carryforward expiration date. Deferred Tax Assets Charitable Contribution Carryforward Expires on December 31, 2021 [Member] Deferred Tax Assets Charitable Contribution Carryforward Expiration Date [Member] Income Tax [Line Items] Income Tax [Line Items] Alternative minimum tax refundable period Tax Cuts And Jobs Act Of2017 Alternative Minimum Tax Refundable Period Income tax refund received Income Tax Examination Liability Refund Adjustment From Settlement With Taxing Authority State [Member] State And Local Jurisdiction [Member] Deferred tax assets: Components Of Deferred Tax Assets [Abstract] Deferred tax assets, gross Deferred Tax Assets Gross Valuation allowance Deferred Tax Assets Valuation Allowance Deferred tax assets, net Deferred Tax Assets Net Deferred tax liabilities: Components Of Deferred Tax Liabilities [Abstract] Deferred tax liabilities, gross Deferred Income Tax Liabilities Net deferred tax asset Deferred Tax Assets Liabilities Net Deferred tax liabilities funded status of post-retirement benefits. Deferred tax assets depreciation and amortization. Employee benefit plans Deferred Tax Assets Tax Deferred Expense Compensation And Benefits Employee Benefits Allowance for loan losses Deferred Tax Assets Tax Deferred Expense Reserves And Accruals Provision For Loan Losses Funded status of post-retirement benefits Deferred Tax Liabilities Funded Status Of Post Retirement Benefits Securities available for sale Deferred Tax Liabilities Investments Depreciation and amortization Deferred Tax Assets Depreciation And Amortization Net deferred loan origination costs Deferred Tax Liabilities Financing Arrangements Mortgage servicing rights Deferred Tax Liabilities Mortgage Servicing Rights Net operating loss carryforward Deferred Tax Assets Operating Loss Carryforwards Charitable contribution carryforward Deferred Tax Assets Charitable Contribution Carryforwards Derivatives Deferred Tax Liabilities Derivatives Stock-based compensation Deferred Tax Assets Tax Deferred Expense Compensation And Benefits Share Based Compensation Cost Other, net Deferred Tax Liabilities Other Deferred tax assets (liabilities) before valuation allowances Deferred Tax Assets Liabilities Before Valuation Allowance Gross Valuation allowance on net deferred tax assets Net operating loss carryforward Operating Loss Carryforwards Operating loss carryforward expiration date Operating Loss Carryforwards Expiration Date Charitable contribution carryforwards. Deferred tax assets charitable contribution carryforward expiration date. Charitable contribution carryforward Charitable Contribution Carryforwards Charitable contributions carryforward expiration years Deferred Tax Assets Charitable Contribution Carryforward Expiration Date Deferred tax assets valuation allowance percentage Valuation allowance for deferred tax assets Deferred Tax Assets Valuation Allowance Percentage Net income Federal income tax reserve for loan losses. Percentage of tax reserve for loan losses used for purpose other than to absorb losses subject to taxation. Federal income tax reserve for loan losses Federal Income Tax Reserve For Loan Losses Percentage of tax reserve for loan losses used for purpose other than to absorb losses subject to taxation Percentage Of Tax Reserve For Loan Losses Used For Purpose Other Than To Absorb Losses Subject To Taxation Deferred income tax liability not been provided Deferred Tax Liability Not Recognized Amount Of Unrecognized Deferred Tax Liability Bad Debt Reserve For Tax Purposes Of Qualified Lender Derivative [Table] Derivative [Table] Derivative loan commitments. Derivative Loan Commitments [Member] Derivative Loan Commitments [Member] Net gain on sales of mortgage loans. Net Gain on Sales of Mortgage Loans [Member] Net Gain On Sales Of Mortgage Loans [Member] Other Liabilities [Member] Other Liabilities [Member] Forward loan sale commitments and to be announced securities. Undesignated Forward Loan Sale Commitments and TBA Securities [Member] Forward Loan Sale Commitments And To Be Announced Securities [Member] Derivative [Line Items] Derivative [Line Items] Derivative, Notional amount Derivative Notional Amount Derivative fair value, Asset Derivative Assets Increase (reductions) in aggregate fair value on derivatives, interest rate lock commitments Derivative Gain Loss On Derivative Net Derivative fair value, Liability Derivative Liabilities Schedule Of Compliance With Regulatory Capital Requirements Under Banking Regulations [Table] Schedule Of Compliance With Regulatory Capital Requirements Under Banking Regulations [Table] Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items] Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items] Capital conservation buffer of common equity tier 1 capital to risk weighted assets ratio (as a percent) Excess Capital To Risk Weighted Assets Common equity tier one capital to risk weighted assets. Legal Entity Legal Entity [Axis] Entity Entity [Domain] Randolph Savings Bank. Bank [Member] Randolph Savings Bank [Member] Total capital (to risk weighted assets), actual amount Capital Tier 1 capital (to risk weighted assets), actual amount Tier One Risk Based Capital Common equity Tier 1 capital (to risk weighted assets), actual amount Common Equity Tier One Capital To Risk Weighted Assets Tier 1 capital (to average assets), actual amount Tier One Leverage Capital Common equity tier one capital to risk weighted assets ratio. Total capital (to risk weighted assets), actual ratio Capital To Risk Weighted Assets Tier 1 capital (to risk weighted assets), actual ratio Tier One Risk Based Capital To Risk Weighted Assets Common equity Tier 1 capital (to risk weighted assets), actual ratio Common Equity Tier One Capital To Risk Weighted Assets Ratio Tier 1 capital (to average assets), actual ratio Tier One Leverage Capital To Average Assets Common equity tier one capital to risk weighted assets minimum capital required for capital adequacy. Total capital (to risk weighted assets), for minimum capital adequacy purposes amount Capital Required For Capital Adequacy Tier 1 capital (to risk weighted assets), for minimum capital adequacy purposes amount Tier One Risk Based Capital Required For Capital Adequacy Common equity Tier 1 capital (to risk weighted assets), for minimum capital adequacy amount Common Equity Tier One Capital To Risk Weighted Assets Minimum Capital Required For Capital Adequacy Common equity Tier 1 capital (to risk weighted assets), for minimum capital adequacy amount Tier One Leverage Capital Required For Capital Adequacy Common equity tier one capital to risk weighted assets minimum capital required for capital adequacy ratio. Total capital (to risk weighted assets), for minimum capital adequacy purposes ratio Capital Required For Capital Adequacy To Risk Weighted Assets Tier 1 capital (to risk weighted assets), for minimum capital adequacy purposes ratio Tier One Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets Common equity Tier 1 capital (to risk weighted assets), for minimum capital adequacy purposes ratio Common Equity Tier One Capital To Risk Weighted Assets Minimum Capital Required For Capital Adequacy Ratio Tier 1 capital (to average assets), for minimum capital adequacy purposes ratio Tier One Leverage Capital Required For Capital Adequacy To Average Assets Common equity tier one capital to risk weighted assets minimum to be well capitalized under prompt corrective action provisions. Total capital (to risk weighted assets), minimum to be well capitalized under prompt corrective action provisions amount Capital Required To Be Well Capitalized Tier 1 capital (to risk weighted assets), minimum to be well capitalized under prompt corrective action provisions amount Tier One Risk Based Capital Required To Be Well Capitalized Common equity Tier 1 capital (to risk weighted assets), minimum to be well capitalized under prompt corrective action provisions amount Common Equity Tier One Capital To Risk Weighted Assets Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions Tier 1 capital (to average assets), minimum to be well capitalized under prompt corrective action provisions amount Tier One Leverage Capital Required To Be Well Capitalized Common equity tier one capital to risk weighted assets minimum to be well capitalized under prompt corrective action provisions ratio. Total capital (to risk weighted assets), minimum to be well capitalized under prompt corrective action provisions ratio Capital Required To Be Well Capitalized To Risk Weighted Assets Tier 1 capital (to risk weighted assets), minimum to be well capitalized under prompt corrective action provisions ratio Tier One Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets Common equity Tier 1 capital (to risk weighted assets), minimum to be well capitalized under prompt corrective action provisions ratio Common Equity Tier One Capital To Risk Weighted Assets Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio Tier 1 capital (to average assets), minimum to be well capitalized under prompt corrective action provisions ratio Tier One Leverage Capital Required To Be Well Capitalized To Average Assets Schedule Of Defined Benefit Plans Disclosures [Table] Schedule Of Defined Benefit Plans Disclosures [Table] Endorsement split-dollar life insurance arrangements. Endorsement Split-Dollar Life Insurance Arrangements [Member] Endorsement Split Dollar Life Insurance Arrangements [Member] Four zero one k plan member. 401(k) Plan [Member] Four Zero One K Plan [Member] Defined Benefit Plan Disclosure [Line Items] Defined Benefit Plan Disclosure [Line Items] Accrued benefits Pension And Other Postretirement Defined Benefit Plans Liabilities Current And Noncurrent Other non-interest expense Change in plan assets: Defined Benefit Plan Change In Fair Value Of Plan Assets Roll Forward Fair value of plan assets at beginning of year Defined Benefit Plan Fair Value Of Plan Assets Employer contributions Defined Benefit Plan Contributions By Employer Benefits paid Defined Benefit Plan Plan Assets Benefits Paid Fair value of plan assets at end of year Change in benefit obligation: Defined Benefit Plan Change In Benefit Obligation Roll Forward Benefit obligation at beginning of year Defined Benefit Plan Benefit Obligation Service cost Defined Benefit Plan Service Cost Interest cost Defined Benefit Plan Interest Cost Actuarial (gain) loss Defined Benefit Plan Actuarial Gain Loss Benefits paid Defined Benefit Plan Benefit Obligation Benefits Paid Benefit obligation at end of year Unfunded status and accrued supplemental pension cost at year end Defined Benefit Plan Funded Status Of Plan Accumulated benefit obligation at year end Defined Benefit Plan Accumulated Benefit Obligation Defined benefit plan assumptions used calculating benefit obligation annual inflation factor rate. Discount rate Defined Benefit Plan Assumptions Used Calculating Benefit Obligation Discount Rate Annual inflation factor Defined Benefit Plan Assumptions Used Calculating Benefit Obligation Annual Inflation Factor Rate Amortization of net actuarial loss Defined Benefit Plan Amortization Of Gains Losses Amortization of prior service credit Defined Benefit Plan Amortization Of Prior Service Cost Credit Net periodic benefit cost Defined Benefit Plan Net Periodic Benefit Cost Defined benefit plan assumptions used calculating net periodic benefit cost annual inflation factor rate. Discount rate Defined Benefit Plan Assumptions Used Calculating Net Periodic Benefit Cost Discount Rate Annual inflation factor Defined Benefit Plan Assumptions Used Calculating Net Periodic Benefit Cost Annual Inflation Factor Rate 2020 Defined Benefit Plan Expected Future Benefit Payments Next Twelve Months 2021 Defined Benefit Plan Expected Future Benefit Payments Year Two 2022 Defined Benefit Plan Expected Future Benefit Payments Year Three 2023 Defined Benefit Plan Expected Future Benefit Payments Year Four 2024 Defined Benefit Plan Expected Future Benefit Payments Year Five 2025-2029 Defined Benefit Plan Expected Future Benefit Payments Five Fiscal Years Thereafter Post-retirement benefit expense Pension And Other Postretirement Benefit Expense Employer matching contributions vesting period. Automatic employer matching contributions vesting period. Employer match of eligible employees contributions percentage. Minimum age of employee to become participant. Minimum number of year worked by employees. Minimum term worked by employees. Description of employees eligibility Defined Benefit Plans General Information Minimum age of employee to become participant Minimum Age Of Employee To Become Participant Maximum annual contributions per employee, percent Defined Contribution Plan Maximum Annual Contributions Per Employee Percent Minimum number of year worked by employees Minimum Number Of Year Worked By Employees Minimum number of hours worked by employees Minimum Term Worked By Employees Employer contribution percentage on employees compensation Defined Contribution Plan Employer Matching Contribution Percent Automatic employer matching contributions vesting period Automatic Employer Matching Contributions Vesting Period Employer matching contribution, percent Defined Contribution Plan Employer Matching Contribution Percent Of Match Employer match of eligible employees contributions percentage Employer Match Of Eligible Employees Contributions Percentage Employer matching contributions vesting period Employer Matching Contributions Vesting Period Contribution expense Defined Contribution Plan Cost Recognized Schedule Of Share Based Compensation Arrangements By Share Based Payment Award [Table] Schedule Of Share Based Compensation Arrangements By Share Based Payment Award [Table] Plan Name Plan Name [Axis] Plan Name Plan Name [Domain] Two thousand seventeen stock option and incentive plan. Two Thousand Seventeen Stock Option and Incentive Plan [Member] Two Thousand Seventeen Stock Option And Incentive Plan [Member] Award Type Award Type [Axis] Award Type Share Based Compensation Arrangements By Share Based Payment Award Award Type And Plan Name [Domain] Stock Options [Member] Employee Stock Option [Member] Restricted Stock [Member] Restricted Stock [Member] Share Based Compensation Arrangement By Share Based Payment Award [Line Items] Share Based Compensation Arrangement By Share Based Payment Award [Line Items] Shares reserved for issuance Share Based Compensation Arrangement By Share Based Payment Award Number Of Shares Authorized Stock options term Sharebased Compensation Arrangement By Sharebased Payment Award Expiration Period Options and awards vesting period Share Based Compensation Arrangement By Share Based Payment Award Award Vesting Period1 Options granted Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Gross Vesting period (years) Expiration period (years) Expected volatility Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Volatility Rate Expected volatility Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Volatility Rate Minimum Expected volatility Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Volatility Rate Maximum Expected life (years) Sharebased Compensation Arrangement By Sharebased Payment Award Fair Value Assumptions Expected Term1 Expected dividend yield Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Dividend Rate Risk free interest rate Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Risk Free Interest Rate Risk free interest rate, minimum Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Risk Free Interest Rate Minimum Risk free interest rate, maximum Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Risk Free Interest Rate Maximum Option fair value Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Weighted Average Grant Date Fair Value Stock Option Grants Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward Beginning Balance Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Number Granted Forfeited Share Based Compensation Arrangement By Share Based Payment Award Options Forfeitures In Period Expired Share Based Compensation Arrangement By Share Based Payment Award Options Expirations In Period Ending Balance Exercisable at December 31, 2019 Share Based Compensation Arrangement By Share Based Payment Award Options Exercisable Number Unrecognized compensation cost (inclusive of directors' options) Employee Service Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized Stock Options Weighted average remaining recognition period (years) Employee Service Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized Period For Recognition1 Weighted Average Exercise Price Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price Rollforward Beginning Balance Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price Granted Share Based Compensation Arrangements By Share Based Payment Award Options Grants In Period Weighted Average Exercise Price Forfeited Share Based Compensation Arrangements By Share Based Payment Award Options Forfeitures In Period Weighted Average Exercise Price Expired Share Based Compensation Arrangements By Share Based Payment Award Options Expirations In Period Weighted Average Exercise Price Ending Balance Exercisable at December 31, 2019 Share Based Compensation Arrangement By Share Based Payment Award Options Exercisable Weighted Average Exercise Price Share-based compensation arrangement by share-based payment award, options, grants in period, weighted average remaining contractual term. Weighted Average Remaining Contractual Term and Aggregate Intrinsic Value Share Based Compensation Arrangement By Share Based Payment Award Options Additional Disclosures [Abstract] Weighted Average Remaining Contractual Term Sharebased Compensation Arrangement By Sharebased Payment Award Options Outstanding Weighted Average Remaining Contractual Term2 Weighted Average Remaining Contractual Term, Exercisable Sharebased Compensation Arrangement By Sharebased Payment Award Options Exercisable Weighted Average Remaining Contractual Term1 Share-based compensation arrangement by Share-based payment award options forfeited intrinsic value. Aggregate Intrinsic Value, Outstanding Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Intrinsic Value Aggregate Intrinsic Value, Granted Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Grant Date Intrinsic Value Aggregate Intrinsic Value, Outstanding Aggregate Intrinsic Value, Exercisable Sharebased Compensation Arrangement By Sharebased Payment Award Options Exercisable Intrinsic Value1 Stock-based compensation expense Allocated Share Based Compensation Expense Restricted Stock Awards Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Roll Forward Beginning Balance Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Number Granted Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Vested Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested In Period Forfeited Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Forfeited In Period Ending Balance Unrecognized compensation cost Employee Service Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized Share Based Awards Other Than Options Weighted Average Grant Price Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Weighted Average Grant Date Fair Value Roll Forward Beginning Balance Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Weighted Average Grant Date Fair Value Granted Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Weighted Average Grant Date Fair Value Vested Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested In Period Weighted Average Grant Date Fair Value Forfeited Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Forfeitures Weighted Average Grant Date Fair Value Ending Balance Term of loan repaid by employee stock ownership plan. Annual allocation of shares expiration year. Employee stock ownership plan number of unallocated shares. Employee stock ownership plan fair value of unallocated shares. Schedule Of Employee Stock Ownership Plan E S O P Disclosures [Table] Schedule Of Employee Stock Ownership Plan E S O P Disclosures [Table] Related Party Related Party Transactions By Related Party [Axis] Related Party Related Party [Domain] The Randolph Savings Charitable Foundation, Inc. The Randolph Savings Charitable Foundation, Inc. [Member] Foundation [Member] Employee Stock Ownership Plan E S O P Disclosures [Line Items] Employee Stock Ownership Plan E S O P Disclosures [Line Items] Sale of share in employee stock ownership plan Stock Issued During Period Shares Employee Stock Ownership Plan Sale of stock, price per share Sale Of Stock Price Per Share Loan repaid term Term Of Loan Repaid By Employee Stock Ownership Plan Interest rate Debt Instrument Interest Rate During Period Number of allocated shares Employee Stock Ownership Plan E S O P Number Of Allocated Shares Annual allocation of shares, expiration year Annual Allocation Of Shares Expiration Year Unallocated shares Employee Stock Ownership Plan Number Of Unallocated Shares Unallocated shares, value Employee Stock Ownership Plan Fair Value Of Unallocated Shares Stock repurchase program percentage of outstanding shares repurchased. Schedule Of Stock By Class [Table] Schedule Of Stock By Class [Table] Share Repurchase Program Share Repurchase Program [Axis] Share Repurchase Program Share Repurchase Program [Domain] September two thousand seventeen share repurchase program. September 2017 Share Repurchase Program [Member] September Two Thousand Seventeen Share Repurchase Program [Member] Class Of Stock [Line Items] Class Of Stock [Line Items] Stock repurchase program percentage of outstanding shares repurchased Stock Repurchase Program Percentage Of Outstanding Shares Repurchased Number of shares authorized to repurchase Stock Repurchase Program Number Of Shares Authorized To Be Repurchased Number of shares repurchased Cost of shares repurchased Potentially dilutive common stock equivalents outstanding Weighted Average Number Diluted Shares Outstanding Adjustment Average number of common shares outstanding Weighted Average Number Of Shares Outstanding Basic Less: Average unallocated ESOP shares Weighted Average Number Of Shares Employee Stock Ownership Plan Shares Not Committed To Be Released Average number of common shares outstanding (basic and diluted) Lessee operating lease liability lease rentals payments due next twelve months. Lessee operating lease liability lease rentals payments due year two. Lessee operating lease liability lease rentals payments due year three. Lessee operating lease liability lease rentals payments due year four. Lessee operating lease liability lease rentals payments due year five. Lessee operating lease liability lease rentals payments due after year five. Lessee operating lease liability lease rentals payments due. Lessee operating lease liability sub-lease rentals payments due. Lessee operating lease liability sub-lease rentals payments due next twelve months. Lessee operating lease liability sub-lease rentals payments due year two. Lessee operating lease liability sub-lease rentals payments due year three. Lessee operating lease liability sub-lease rentals payments due year four. Lessee operating lease liability sub-lease rentals payments due year five. Lessee operating lease liability sub-lease rentals payments due after year five. Operating Lease Liabilities Payments Due [Abstract] 2020 Lessee Operating Lease Liability Lease Rentals Payments Due Next Twelve Months 2021 Lessee Operating Lease Liability Lease Rentals Payments Due Year Two 2022 Lessee Operating Lease Liability Lease Rentals Payments Due Year Three 2023 Lessee Operating Lease Liability Lease Rentals Payments Due Year Four 2024 Lessee Operating Lease Liability Lease Rentals Payments Due Year Five Thereafter Lessee Operating Lease Liability Lease Rentals Payments Due After Year Five Future minimum rent commitments, Leases Lessee Operating Lease Liability Lease Rentals Payments Due 2020 Lessee Operating Lease Liability Sub Lease Rentals Payments Due Next Twelve Months 2021 Lessee Operating Lease Liability Sub Lease Rentals Payments Due Year Two 2022 Lessee Operating Lease Liability Sub Lease Rentals Payments Due Year Three 2023 Lessee Operating Lease Liability Sub Lease Rentals Payments Due Year Four 2024 Lessee Operating Lease Liability Sub Lease Rentals Payments Due Year Five Thereafter Lessee Operating Lease Liability Sub Lease Rentals Payments Due After Year Five Future minimum rent commitments, Sub-leases Lessee Operating Lease Liability Sub Lease Rentals Payments Due 2020 Lessee Operating Lease Liability Payments Due Next Twelve Months 2021 Lessee Operating Lease Liability Payments Due Year Two 2022 Lessee Operating Lease Liability Payments Due Year Three 2023 Lessee Operating Lease Liability Payments Due Year Four 2024 Lessee Operating Lease Liability Payments Due Year Five Thereafter Lessee Operating Lease Liability Payments Due After Year Five Future minimum rent commitments, Net Lessee Operating Lease Liability Payments Due Commitments and contingencies. Commitments and contingencies. Commitments And Contingencies [Table] Commitments And Contingencies [Table] Title of Individual Title Of Individual [Axis] Title of Individual Title Of Individual With Relationship To Entity [Domain] Senior management, Senior Management [Member] Senior Management [Member] Commitments And Contingencies [Line Items] Commitments And Contingencies [Line Items] Lease options to extend the period Lessee Operating Lease Renewal Term Rent expense Lease And Rental Expense Schedule Of Fair Value Off Balance Sheet Risks [Table] Schedule Of Fair Value Off Balance Sheet Risks [Table] Commitments to Originate Loans [Member] Loan Origination Commitments [Member] Unused lines and letters of credit. Unused Lines and Letters of Credit [Member] Unused Lines And Letters Of Credit [Member] Unadvanced funds on construction loans. Unadvanced Funds on Construction Loans [Member] Unadvanced Funds On Construction Loans [Member] Overdraft. Overdraft Lines of Credit [Member] Overdraft [Member] Fair Value Off Balance Sheet Risks Disclosure Information [Line Items] Fair Value Off Balance Sheet Risks Disclosure Information [Line Items] Financial instruments with off-balance-sheet risk Fair Value Disclosure Offbalance Sheet Risks Face Amount Liability Salary continuation term of employment arrangement. Salary continuation term of employment arrangement Salary Continuation Term Of Employment Arrangement Number of members with change in control agreements. Change in control agreement description of terms. Number of members with change in control agreements Number Of Members With Change In Control Agreements Change in control agreement description of terms Change In Control Agreement Description Of Terms Portfolio loans fair value disclosure. Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Table] Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Table] Measurement Frequency Fair Value By Measurement Frequency [Axis] Measurement Frequency Fair Value Measurement Frequency [Domain] Fair Value, Measurements, Recurring [Member] Fair Value Measurements Recurring [Member] Fair Value Hierarchy and NAV Fair Value By Fair Value Hierarchy Level [Axis] Fair Value Hierarchy and NAV Fair Value Measurements Fair Value Hierarchy [Domain] Level 1 [Member] Fair Value Inputs Level1 [Member] Level 2 [Member] Fair Value Inputs Level2 [Member] Level 3 [Member] Fair Value Inputs Level3 [Member] Forward loan sale commitments. Forward Loan Sale Commitments [Member] Forward Loan Sale Commitments [Member] Forward loan sale commitments, including TBAs. Forward Loan Sale Commitments, including TBAs [Member] Forward Loan Sale Commitments Including T B As [Member] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] Fair value, Portfolio loans Portfolio Loans Fair Value Disclosure Fair value, Loans held for sale Loans Held For Sale Fair Value Disclosure Derivative assets Derivative liabilities Fair Value, Nonrecurring Fair Value Measurements Nonrecurring [Member] Fair value, assets, level 1 to level 2 transfers, amount Fair Value Assets Level1 To Level2 Transfers Amount Fair value, assets, level 2 to level 1 transfers, amount Fair Value Assets Level2 To Level1 Transfers Amount Fair value, liabilities, level 1 to level 2 transfers, amount Fair Value Liabilities Level1 To Level2 Transfers Amount Fair value, liabilities, level 2 to level 1 transfers, amount Fair Value Liabilities Level2 To Level1 Transfers Amount Liabilities measured at fair value on nonrecurring basis Liabilities Fair Value Disclosure Gains losses on assets held. Fair value, Collateral dependent impaired loans Collateral Dependent Impaired Loans Fair Value Mortgage servicing rights, fair value Fair value, Foreclosed real estate Real Estate Acquired Through Foreclosure Fair Value Assets, Fair Value Disclosure Assets Fair Value Disclosure Gains (Losses) on collateral dependent impaired loans Gain Loss On Sale Of Collateral Dependent Impaired Loans Gains (losses) on assets held, mortgage servicing rights Gains Losses On Assets Held Mortgage Servicing Rights Gains (losses) on assets held, foreclosed real estate Gains Losses On Assets Held Foreclosed Real Estate Gains (Losses) on assets held Gains Losses On Assets Held Certificate of deposits fair value disclosure. Measurement Basis Fair Value By Measurement Basis [Axis] Fair Value Disclosure Item Amounts [Default] Portion At Fair Value Fair Value Disclosure [Member] Fair Value Measurement Fair Value Disclosure Item Amounts [Domain] Carrying Amount [Member] Carrying Reported Amount Fair Value Disclosure [Member] Fair Value [Member] Estimate Of Fair Value Fair Value Disclosure [Member] Financial assets: Financial Instruments Financial Assets Balance Sheet Groupings [Abstract] Certificates of deposit Certificate Of Deposits Fair Value Disclosure Securities available for sale Loans held for sale Loans, net Loans Receivable Fair Value Disclosure Financial liabilities: Financial Instruments Financial Liabilities Balance Sheet Groupings [Abstract] Deposits Deposits Fair Value Disclosure FHLBB advances Federal Home Loan Bank Borrowings Fair Value Disclosure Number of branch offices. Schedule Of Segment Reporting Information By Segment [Table] Schedule Of Segment Reporting Information By Segment [Table] Segments Statement Business Segments [Axis] Segments Segment [Domain] Envision Mortgage. Envision Mortgage [Member] Envision Mortgage [Member] Envision Bank. Envision Bank [Member] Envision Bank [Member] Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Number of branch offices Number Of Branch Offices Noninterest income excluding intercompany profit. Net interest income Provision for loan losses Gain on loan origination and sale activities, net Gain On Loan Origination And Sale Activities Net Excluding Intercompany Profit Gain on sales of buildings Gains Losses On Sales Of Assets Other Segment Other Noninterest Income Total non-interest income Noninterest Income Excluding Intercompany Profit Merger and integration costs Merger And Integration Cost Other non-interest expenses Segment Other Noninterest Expense Income (loss) before income taxes and elimination of inter-segment profit Income Loss Before Income Taxes And Elimination Of Intersegment Profit Elimination of inter-segment profit Elimination Of Intersegment Profit Total assets Premium percentage for new loans. Percentage fees for HELOC origination. Premium percentage for new loans Premium Percentage For New Loans Premium income Premiums Earned Net Percentage of fees for HELOC Percentage Fees For H E L O C Origination Percentage of loan servicing fees. Percentage of loan servicing fees Percentage Of Loan Servicing Fees Servicing fees amount Percentage of certain items exceeding interest and non interest income. Percentage of certain items exceeding interest and non interest income Percentage Of Certain Items Exceeding Interest And Non Interest Income Software amortization, licenses and maintenance. Card related expenses. Data communication and telephone expense. Noninterest expense directors fees including stock based compensation. Software amortization, licenses and maintenance Software Amortization Licenses And Maintenance Card related expenses Card Related Expenses Data communication and telephone Data Communication And Telephone Expense Directors fees, including stock-based compensation Noninterest Expense Directors Fees Including Stock Based Compensation The number of positions to be added during the period as a result of restructuring activities. Restructuring and related cost number of employees maintained. Schedule Of Restructuring And Related Costs [Table] Schedule Of Restructuring And Related Costs [Table] North Attleboro. North Attleboro [Member] North Attleboro [Member] Restructuring Cost And Reserve [Line Items] Restructuring Cost And Reserve [Line Items] Number of administrative positions eliminated Restructuring And Related Cost Number Of Positions Eliminated Number of administrative positions to be added Restructuring And Related Cost Number Of Positions To Be Added Number of employees remained Restructuring And Related Cost Number Of Employees Maintained Severance costs Severance Costs1 Remaining space available. Remaining space available Remaining Space Available Impairment write-down on furniture and equipment Condensed Balance Sheet Statement [Table] Condensed Balance Sheet Statement [Table] Consolidated Entities Consolidated Entities [Axis] Consolidated Entities Consolidated Entities [Domain] Randolph Bancorp, Inc. and Predecessor Randolph Bancorp [Member] Parent Company [Member] Condensed Balance Sheet Statements Captions [Line Items] Condensed Balance Sheet Statements Captions [Line Items] Investment in Envision Bank Investments In Affiliates Subsidiaries Associates And Joint Ventures ESOP loan Employee Stock Ownership Plan Loan Liabilities Liabilities [Abstract] Accounts payable Accounts Payable Current And Noncurrent Due to Envision Bank Due To Affiliate Current And Noncurrent Common stock Condensed Income Statement [Table] Condensed Income Statement [Table] Condensed Income Statements Captions [Line Items] Condensed Income Statements Captions [Line Items] Interest income Operating expenses Operating Expenses Income before incomes taxes and equity in undistributed net income (loss) of Envision Bank Income Loss From Continuing Operations Before Income Taxes Minority Interest And Income Loss From Equity Method Investments Income before equity in undistributed net income (loss) of Envision Bank Income Loss From Continuing Operations Net Of Tax Before Equity Method Investments Equity in undistributed net income (loss) of Envision Bank Income Loss From Equity Method Investments Net Of Dividends Or Distributions Condensed Cash Flow Statement [Table] Condensed Cash Flow Statement [Table] Condensed Cash Flow Statements Captions [Line Items] Condensed Cash Flow Statements Captions [Line Items] Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities: Equity in undistributed net (income) loss of Envision Bank Change in intercompany receivable/payable Increase Decrease In Intercompany Receivable Principal payments received on ESOP loan Proceeds From Employee Stock Ownership Plans Loans Investing Activities Stock repurchases Stock repurchase payable Proceeds From Repurchase Of Equity Interest and dividend income Interest expense Gain on loan origination and sales activities, net Gain Loss On Sales Of Loans Net Other non-interest income Noninterest Income Other Operating Income1 Other non-interest expense Other Noninterest Expense1 Provision (benefit) for income taxes Basic and diluted earnings (loss) per share Weighted average common shares (basic and diluted) Schedule Of Effect Of Fourth Quarter Events [Table] Schedule Of Effect Of Fourth Quarter Events [Table] Other non-interest income member. Other Non Interest Income [Member] Other Non Interest Income [Member] Business Acquisition Business Acquisition [Axis] Business Acquisition, Acquiree Business Acquisition Acquiree [Domain] Boston Branch Location [Member] Effect Of Fourth Quarter Events [Line Items] Effect Of Fourth Quarter Events [Line Items] Increase of valuation allowance for mortgage servicing rights Gain recognized on sale of former branch location Gain Loss On Sale Of Business Supplemental unemployment benefits salary continuation period. Supplemental unemployment benefits severance benefits period. President And Chief Executive Officer. President and Chief Executive Officer [Member] President And Chief Executive Officer [Member] Scenario Statement Scenario [Axis] Scenario Scenario Unspecified [Domain] Scenario Forecast [Member] Scenario Forecast [Member] Current base salary Supplemental Unemployment Benefits Salary Continuation Current base salary, period Supplemental Unemployment Benefits Salary Continuation Period Transition Salary payable Supplemental Unemployment Benefits Severance Benefits Transition Salary payable, period Supplemental Unemployment Benefits Severance Benefits Period Recognize compensation expense Deferred Compensation Arrangement With Individual Compensation Expense