Investments, Debt and Equity Securities [Abstract] Available for Sale Securities Schedule of Available-for-sale Securities Reconciliation [Table Text Block] Held-to-maturity Securities Debt Securities, Held-to-maturity [Table Text Block] Amortized Cost and Fair Value of Debt Securities Schedule Of Amortized Cost And Fair Value Of Debt Securities Table [Table Text Block] Schedule Of Amortized Cost And Fair Value Of Debt Securities. Summary of Gross Unrealized Losses and Fair Value Schedule of Unrealized Loss on Investments [Table Text Block] Fair Value Disclosures [Abstract] Fair Value, by Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping [Table] Measurement Basis [Axis] Measurement Basis [Axis] Fair Value Measurement [Domain] Fair Value Measurement [Domain] Portion at Fair Value Measurement Portion at Fair Value Measurement [Member] Carrying Value Reported Value Measurement [Member] Fair Value Estimate of Fair Value Measurement [Member] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Quoted Prices in Active Markets for Identical Assets (Level 1) Fair Value, Inputs, Level 1 [Member] Other Observable Inputs (Level 2) Fair Value, Inputs, Level 2 [Member] Significant Unobservable Inputs (Level 3) Fair Value, Inputs, Level 3 [Member] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Financial assets: Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] Cash and cash equivalents Cash and Cash Equivalents, Fair Value Disclosure Available-for-sale securities Debt Securities, Available-for-sale Available-for-sale securities Available-for-sale Securities Held-to-maturity securities Debt Securities, Held-to-maturity, Fair Value Loans held for sale Loans Held-for-sale, Fair Value Disclosure Loans receivable-net Loans Receivable, Fair Value Disclosure FHLBB stock Investment in Federal Home Loan Bank Stock, Fair Value Disclosure Accrued interest receivable Accrued Interest Receivable Fair Value Disclosure Accrued interest receivable fair value disclosure. Derivative assets Mortgage Loans Derivative Assets Mortgage loans derivative assets. Mortgage servicing rights Mortgage Servicing Rights Fair Value Disclosure Mortgage servicing rights. Marketable equity securities Equity Securities, FV-NI Financial liabilities: Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract] Deposits Deposits, Fair Value Disclosure Mortgagors’ and investors’ escrow accounts Mortgagors And Investors Escrow Accounts Fair Value Disclosure Mortgagors and investors escrow accounts fair value disclosure. FHLBB advances and other borrowings Advances from Federal Home Loan Banks Derivative liabilities Derivative Liability Accounting Policies [Abstract] Nature of Operations Nature Of Operations Policy [Policy Text Block] Nature of operations. Basis of Presentation Basis of Accounting, Policy [Policy Text Block] Common Share Repurchases Common Stock Share Repurchases Policy [Policy Text Block] Common Stock Share Repurchases [Policy Text Block] Reclassifications Reclassification, Policy [Policy Text Block] Use of Estimates Use of Estimates, Policy [Policy Text Block] Recent Accounting Pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Banking and Thrift [Abstract] Schedule of Federal Home Loan Bank Advances, by Branch of FHLB Bank [Table] Schedule of Federal Home Loan Bank Advances, by Branch of FHLB Bank [Table] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Weighted Average Weighted Average [Member] Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items] Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items] Federal Home Loan Bank, Advances, Fiscal Year Maturity [Abstract] Federal Home Loan Bank, Advances, Fiscal Year Maturity [Abstract] 2018 Federal Home Loan Bank, Advances, Maturities Summary, Due in Remainder of Fiscal Year 2019 Federal Home Loan Bank, Advances, Maturities Summary, Due in Year Two 2020 Federal Home Loan Bank, Advances, Maturities Summary, Due in Year Three 2021 Federal Home Loan Bank, Advances, Maturities Summary, Due in Year Four 2022 Federal Home Loan Bank, Advances, Maturities Summary, Due in Year Five Thereafter Federal Home Loan Bank, Advances, Maturities Summary, Due after Year Five FHLBB, advances, total Federal Home Loan Bank Advances And Other Borrowings Federal Home Loan Bank Advances And Other Borrowings Weighted-Average rate in 2018 Federal Home Loan Bank Advances Branch Of Fhlb Bank Weighted Average Interest Rate Due In Remainder Of Fiscal Year Federal home loan bank advances branch of FHLB bank weighted average interest rate due in remainder of fiscal year. Weighted-Average rate in 2019 Federal Home Loan Bank Advances Branch Of Fhlb Bank Weighted Average Interest Rate Due From One To Two Years Federal home loan bank advances branch of FHLB bank weighted average interest rate due from one to two years. Weighted-Average rate in 2020 Federal Home Loan Bank Advances Branch Of Fhlb Bank Weighted Average Interest Rate Due From Two To Three Years Federal home loan bank advances branch of FHLB bank weighted average interest rate due from two to three years. Weighted-Average rate in 2021 Federal Home Loan Bank Advances Branch Of Fhlb Bank Weighted Average Interest Rate Due From Three To Four Years Federal home loan bank advances branch of FHLB bank weighted average interest rate due from three to four years. Weighted-Average rate in 2022 Federal Home Loan Bank Advances Branch Of FHLB Bank Weighted Average Interest Rate Due From Four To Five Years Federal Home Loan Bank Advances Branch Of FHLB Bank Weighted Average Interest Rate Due From Four To Five Years Weighted-Average rate thereafter Federal Home Loan Bank Advances Branch Of Fhlb Bank Weighted Average Interest Rate Due After Five Years Federal home loan bank advances branch of FHLB bank weighted average interest rate due after five years. FHLBB, advances, branch of FHLB bank, interest rate Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate Disclosure of Compensation Related Costs, Share-based Payments [Abstract] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Award Type [Axis] Award Type [Axis] Equity Award [Domain] Equity Award [Domain] Restricted Stock Restricted Stock [Member] Plan Name [Axis] Plan Name [Axis] Plan Name [Domain] Plan Name [Domain] 2015 Omnibus Stock Incentive Plan Two Thousand Fifteen Omnibus Stock Incentive Plan [Member] Two Thousand Fifteen Omnibus Stock Incentive Plan [Member] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Restricted stock issued (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Weighted average grant date fair value (in usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value Unvested restricted stock, period of recognition Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition Total unrecognized compensation cost related to unvested restricted stock Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized Fair Value Measurements, Recurring and Nonrecurring [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Recurring Fair Value, Measurements, Recurring [Member] Financial Instrument [Axis] Financial Instrument [Axis] Financial Instruments [Domain] Financial Instruments [Domain] Government-sponsored residential mortgage-backed securities Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] Government-sponsored residential collateralized debt obligations US Government-sponsored Enterprises Debt Securities [Member] Government-sponsored commercial mortgage-backed securities Government Sponsored Commercial Mortgage Backed Securities [Member] Government sponsored commercial mortgage backed securities. Government-sponsored commercial collateralized debt obligations Government Sponsored Commercial Collateralized Debt Obligations [Member] Government sponsored commercial collateralized debt obligations. Asset-backed securities Asset-backed Securities [Member] Corporate debt securities Corporate Debt Securities [Member] Obligations of states and political subdivisions US States and Political Subdivisions Debt Securities [Member] Marketable equity securities Equity Securities [Member] Mortgage servicing rights Mortgage Loan Servicing Rights [Member] Mortgage Loan Servicing Rights [Member] Interest rate swaps Interest Rate Swap [Member] SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Loan Type [Axis] SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Loan Type [Axis] SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Loan Type [Domain] SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate, Loan Type [Domain] Mortgage loan derivative assets and liabilities Conventional Mortgage Loan [Member] Available-for-Sale Securities: Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Total available-for-sale debt securities Marketable equity securities Available-for-sale Securities, Equity Securities Mortgage loan derivative assets Derivative Asset Mortgage loan derivative liabilities Loans held for sale Loans Receivable Held-for-sale, Amount Mortgage servicing rights Servicing Asset at Fair Value, Amount Interest rate swap assets Interest Rate Derivative Assets, at Fair Value Interest rate swap liabilities Interest Rate Derivative Liabilities, at Fair Value Receivables [Abstract] Schedule of Impaired Financing Receivable [Table] Schedule of Impaired Financing Receivable [Table] Financing Receivable Portfolio Segment [Axis] Financing Receivable Portfolio Segment [Axis] Financing Receivable Portfolio Segment [Domain] Financing Receivable Portfolio Segment [Domain] Commercial real estate loans Commercial Real Estate Portfolio Segment [Member] Commercial real estate and consumer Commercial Real Estate and Consumer Portfolio Segment [Member] Commercial Real Estate and Consumer Portfolio Segment [Member] Commercial business loans Commercial Portfolio Segment [Member] Consumer loans Consumer Portfolio Segment [Member] Unallocated Unallocated Financing Receivables [Member] Credit Loss Status [Axis] Credit Loss Status [Axis] Credit Loss Status [Domain] Credit Loss Status [Domain] Loans acquired with deteriorated credit quality Financial Asset Acquired with Credit Deterioration [Member] Class of Financing Receivable, Type [Axis] Class of Financing Receivable, Type [Axis] Class of Financing Receivable, Type [Domain] Class of Financing Receivable, Type [Domain] Owner-occupied Owner Occupied Real Estate Loan [Member] Owner Occupied Real Estate Loan [Member] Investor CRE Investor Real Estate Loan [Member] Investor Real Estate Loan [Member] Construction Construction Loans [Member] Commercial business loans Commercial Loan [Member] Residential real estate Residential Real Estate Loan [Member] Residential Real Estate Loan [Member] Home equity Home Equity Loan [Member] Other consumer Other Consumer Loan [Member] Other Consumer Loan [Member] Financing Receivable, Allowance for Credit Losses [Line Items] Financing Receivable, Allowance for Credit Losses [Line Items] Allowance related to loans individually evaluated and deemed impaired Impaired Financing Receivable, Related Allowance Allowance related to loans collectively evaluated and not deemed impaired Financial Receivables Not Deemed To Be Impaired Related Allowance Financial Receivables Not Deemed To Be Impaired Related Allowance. Total allowance for loan losses Loans and Leases Receivable, Allowance Loans deemed impaired Impaired Financing Receivable, Recorded Investment Loans not deemed impaired Financing Receivable Not Deemed To Be Impaired Recorded Investment Financing receivable not deemed to be impaired recorded investment. Loans acquired with deteriorated credit quality Financing Receivable, Net Total loans Loans and Leases Receivable, Gross Nonrecurring Fair Value, Measurements, Nonrecurring [Member] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Impaired loans Impaired Loans Fair Value Disclosure Impaired loans fair value disclosure. Other real estate owned Other Real Estate Owned Fair Value Disclosure Other real estate owned fair value disclosure. Total Assets, Fair Value Disclosure Gains (Losses) in Fair Value Related to Mortgage Loans Held for Sale Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Table Text Block] Schedule of Assets and Liabilities Recorded at Fair Value on Recurring Basis Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Schedule of Assets Measured at Fair Value on Recurring Basis Using Level 3 Inputs Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Quantitative Information of Assets Measured at Fair Value on a Recurring Basis Fair Value Measurement Inputs and Valuation Techniques [Table Text Block] Summary of Assets Recorded at Fair Value on Non-Recurring Basis Fair Value Measurements, Nonrecurring [Table Text Block] Summary of Gains (Losses) on Assets Recorded at Fair Value on Non-Recurring Basis Gains Losses Included In Earnings Fair Value Disclosure Non Recurring Basis Table [Table Text Block] Gains Losses Included In Earnings Fair Value Disclosure Non Recurring Basis Table [Text Block] Summary of Carrying Value and Estimated Fair Values of Financial Instruments Fair Value, by Balance Sheet Grouping [Table Text Block] Summary of Significant Accounting Policies Organization, Consolidation and Presentation of Financial Statements Disclosure and Significant Accounting Policies [Text Block] Fair Value Measurement Inputs and Valuation Techniques [Table] Fair Value Measurement Inputs and Valuation Techniques [Table] Federal Home Loan Bank, Advances, Option [Axis] Federal Home Loan Bank, Advances, Option [Axis] Federal Home Loan Bank, Advances, Option [Domain] Federal Home Loan Bank, Advances, Option [Domain] Federal Home Loan Bank, Advances, Callable Option Federal Home Loan Bank, Advances, Callable Option [Member] Maximum Maximum [Member] Long-term Debt, Type [Axis] Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Long-term Debt, Type [Domain] Subordinated Debt Subordinated Debt [Member] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Subordinated Notes Due October 2024 Subordinated Notes Due October 2024 [Member] Subordinated Notes Due October 2024 [Member] Wholesale Reverse Repurchase Agreements Wholesale Reverse Repurchase Agreements [Member] Wholesale Reverse Repurchase Agreements [Member] Retail Repurchase Agreements Retail Repurchase Agreements [Member] Retail Repurchase Agreements [Member] Brokered Sweep Deposit Brokered Deposits [Member] Brokered Deposits [Member] Variable Rate [Axis] Variable Rate [Axis] Variable Rate [Domain] Variable Rate [Domain] London Interbank Offered Rate (LIBOR) London Interbank Offered Rate (LIBOR) [Member] US Treasury and Government US Treasury and Government [Member] Fair Value, Option, Qualitative Disclosures Related to Election [Abstract] Fair Value, Option, Qualitative Disclosures Related to Election [Abstract] Advances from FHLBB Long-term Federal Home Loan Bank Advances Fair value adjustment on FHLBB advances acquired in the Merger Federal Home Loan Bank, Advances, Valuation Adjustments under Fair Value Option Number of advances Number Of Advances Number of advances. Estimated eligible collateral value Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged Unsecured line of credit Federal Home Loan Bank, Advances, General Debt Obligations, Amount of Available, Unused Funds Additional borrowings Line Of Credit Facility Additional Borrowing Capacity Line of credit facility additional borrowing capacity. Subordinated notes, face amount Debt Instrument, Face Amount Subordinated notes, stated interest rate Debt Instrument, Interest Rate, Stated Percentage Debt issuance cost Debt Issuance Costs, Gross Subordinated notes, carrying value Long-term Debt Junior subordinated debt, face amount Subordinated Debt Fair value acquisition discount Debt Instrument, Unamortized Discount Basis spread on LIBOR rate Debt Instrument, Basis Spread on Variable Rate Other borrowings Other Borrowings Number of individual borrowings, wholesale reverse purchase agreements Number Of Borrowings, Wholesale Reverse Purchase Agreements Number Of Borrowings, Wholesale Reverse Purchase Agreements Other borrowings remaining maturity term (or less) Debt Instrument Remaining Term Debt Instrument Remaining Term Weighted average interest rate Debt, Weighted Average Interest Rate Investment securities pledged as collateral Securities Held as Collateral, at Fair Value Number of leased banking branches with capital lease obligations Number Of Leased Banking Branches With Capital Lease Obligations Number of Leased Banking Branches with Capital Lease Obligations Capital lease obligations Capital Lease Obligations Other borrowings Brokered Deposit Brokered Deposit Percentage of other borrowings at cost Debt Instrument, Interest Rate, Effective Percentage Number of counterparties to unused federal funds lines of credit Number Of Counterparties, Lines Of Credit Number Of Counterparties, Lines Of Credit Committed federal funds lines of credit Line of Credit Facility, Maximum Borrowing Capacity Income Statement [Abstract] Interest and dividend income: Interest and Dividend Income, Operating [Abstract] Loans Interest and Fee Income, Loans and Leases Securities - taxable interest Interest Income, Securities, Operating, Taxable Securities - non-taxable interest Interest Income, Securities, Operating, Tax Exempt Securities - dividends Dividend Income, Operating Interest-bearing deposits Interest Income, Deposits with Financial Institutions Total interest and dividend income Interest and Dividend Income, Operating Interest expense: Interest Expense [Abstract] Deposits Interest Expense, Deposits Borrowed funds Interest Expense, Borrowings 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] Service charges and fees Service Charges And Fees Revenue Service Charges And Fees Revenue (Loss) gain on sales of securities, net Debt and Equity Securities, Gain (Loss) Income from mortgage banking activities Gain (Loss) on Sales of Loans, Net Bank-owned life insurance income Bank Owned Life Insurance, Including Income From Death Benefit Bank Owned Life Insurance, Including Income From Death Benefit Net loss on limited partnership investments Income (Loss) From Limited Partnership Investments, Net Of Tax Income (Loss) From Limited Partnership Investments, Net Of Tax Other income Non Interest Income Expense Other Non Interest Income Expense Other Total non-interest income Noninterest Income Non-interest expense: Noninterest Expense [Abstract] Salaries and employee benefits Labor and Related Expense Service bureau fees Service Bureau Fees Service bureau fees. Occupancy and equipment Occupancy, Net Professional fees Professional Fees Marketing and promotions Marketing and Advertising Expense FDIC insurance assessments Federal Deposit Insurance Corporation Premium Expense Core deposit intangible amortization Amortization of Intangible Assets Other Other Noninterest Expense Total non-interest expense Noninterest Expense Income before income taxes Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest Provision for income taxes Income Tax Expense (Benefit) Net income Net Income (Loss) Attributable to Parent Net income per share: Earnings Per Share, Basic and Diluted [Abstract] Basic (in usd per share) Earnings Per Share, Basic Diluted (in usd per share) Earnings Per Share, Diluted Weighted-average shares outstanding: Weighted Average Number of Shares Outstanding, Diluted [Abstract] Basic (in shares) Weighted Average Number of Shares Outstanding, Basic Diluted (in shares) Weighted Average Number of Shares Outstanding, Diluted Schedule of Accounts, Notes, Loans and Financing Receivable [Table] Schedule of Accounts, Notes, Loans and Financing Receivable [Table] Troubled Debt Restructurings Troubled Debt Restructurings [Member] Troubled Debt Restructurings [Member] Accounts, Notes, Loans and Financing Receivable [Line Items] Accounts, Notes, Loans and Financing Receivable [Line Items] Recorded investment in TDRs: Financing Receivable Recorded Investment Troubled Debt Restructurings [Abstract] Financing Receivable Recorded Investment Troubled Debt Restructurings [Abstract] Accrual status Financing Receivable, Recorded Investment, Current Non-accrual status Financing Receivable, Recorded Investment, Nonaccrual Status Total recorded investment in TDRs Financing Receivable, Modifications, Recorded Investment Accruing TDRs performing under modified terms more than one year Tdr Loans Performing In Accordance With Restructured Terms For More Than One Year Tdr Loans Performing In Accordance With Restructured Terms For More Than One Year Specific reserves for TDRs included in the balance of allowance for loan losses Allowance For Loan And Lease Losses Related To Troubled Debt Restructurings Allowance For Loan And Lease Losses Related To Troubled Debt Restructurings Additional funds committed to borrowers in TDR status Additional Funds Committed To Borrowers In Troubled Debt Restructuring Status Additional Funds Committed To Borrowers In Troubled Debt Restructuring Status Equity [Abstract] Accumulated Other Comprehensive Loss Comprehensive Income (Loss) Note [Text Block] Summary of Company's Loan Portfolio Schedule of Accounts, Notes, Loans and Financing Receivable [Table Text Block] Company's Loans by Risk Rating Financing Receivable Credit Quality Indicators [Table Text Block] Summary of Activity in Allowance for Loan Losses Schedule Of Allowance For Loans By Loan Portfolio [Table Text Block] Schedule of allowance for loans by loan portfolio. Summary of Allowance for Loan Losses and Impaired Loans Schedule Of Allowance For Loan Losses And Impaired Loans [Table Text Block] Schedule Of Allowance for Loan and Lease Losses And Impaired Loans. Summary of Past Due and Non-Accrual Loans Past Due Financing Receivables [Table Text Block] Summary of Impaired Loans with and without Valuation Allowance Impaired Financing Receivables [Table Text Block] Average Recorded Investment in Impaired Loans Summary Of Average Recorded Investment In Impaired Loans With And Without Valuation Allowance And Interest Income Recognized [Table Text Block] Summary of average recorded investment in impaired loans with and without valuation allowance and interest income recognized. Troubled Debt Restructurings Schedule of Debtor Troubled Debt Restructuring, Current Period [Table Text Block] Loans Restructured as Troubled Debt Restructurings Troubled Debt Restructurings on Financing Receivables [Table Text Block] Summary of How Loans were Modified as TDRs Schedule Of Loans Modified As Troubled Debt Restructuring [Table Text Block] Schedule Of Loans Modified As Troubled Debt Restructuring Table [Text Block] Summary of Loans Modified as TDRs within Previous 12 Months and Payment Default Financing Receivables Modified As Troubled Debt Restructurings With Payment Default [Table Text Block] Financing Receivables Modified As Troubled Debt Restructurings With Payment Default Table [Text Block] Summary of Mortgage Servicing Rights Activity Schedule of Servicing Assets at Fair Value [Table Text Block] Regulatory Capital Amounts and Ratios Schedule of Compliance with Regulatory Capital Requirements under Banking Regulations [Table Text Block] Contractual Maturities and Weighted-Average Rates of Outstanding Advances Federal Home Loan Bank, Advances [Table Text Block] Schedule of Other Borrowings by Category Schedule of Debt [Table Text Block] Schedule of Outstanding Borrowings Under Repurchase Agreements Schedule of Repurchase Agreements [Table Text Block] Mortgage servicing rights: Servicing Asset at Fair Value, Amount [Roll Forward] Fair value of mortgage servicing at beginning of period Change in fair value recognized in net income Servicing Asset at Fair Value, Changes in Fair Value Resulting from Changes in Valuation Inputs or Changes in Assumptions Issuances Servicing Asset at Fair Value, Additions Fair value of mortgage servicing rights at end of period Employee Stock Option Employee Stock Option [Member] Options and Restricted Stock Options And Restricted Stock [Member] Options And Restricted Stock [Member] Title of Individual [Axis] Title of Individual [Axis] Relationship to Entity [Domain] Relationship to Entity [Domain] Director Director [Member] Officer Officer [Member] Shares authorized under the plan (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized Share-based awards, fungible ratio Share-based Compensation Arrangement by Share-based Payment Award, Fungible Ratio Share-based Compensation Arrangement by Share-based Payment Award, Fungible Ratio Shares available for future grants (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant Stock-based compensation expense Allocated Share-based Compensation Expense Tax benefit recorded Employee Service Share-based Compensation, Tax Benefit from Compensation Expense Other non-interest expenses Earnings Per Share [Abstract] Net income available to common stockholders Weighted-average common shares outstanding (in shares) Weighted Average Number of Shares Issued, Basic Less: average number of unallocated ESOP award shares (in shares) Weighted Average Number of Shares, Employee Stock Ownership Plan Shares Not Committed to be Released Weighted-average basic shares outstanding (in shares) Dilutive effect of stock options (in shares) Dilutive Effect Of Stock Options Dilutive effect of stock options. Weighted-average diluted shares (in shares) Anti-dilutive stock options excluded from diluted earnings per share calculation (in shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Number of banking offices Number Of Banking Offices Number of banking offices. Number of ATMs Number Of Automated Teller Machines Number of automated teller machines. Recent Accounting Pronouncements New Accounting Pronouncements and Changes in Accounting Principles [Text Block] Activity Related to Stock Options Share-based Compensation, Stock Options, Activity [Table Text Block] Activity for Restricted Stock Schedule of Share-based Compensation, Restricted Stock Units Award Activity [Table Text Block] ESOP Shares Employee Stock Ownership Plan (ESOP) Disclosures [Table Text Block] Statement of Financial Position [Abstract] ASSETS Assets [Abstract] Cash and due from banks Cash and Due from Banks Short-term investments Short-term Investments Total cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Held-to-maturity securities - at amortized cost Debt Securities, Held-to-maturity Loans held for sale Loans Receivable Held-for-sale, Net, Not Part of Disposal Group Loans receivable (net of allowance for loan losses of $49,909 at September 30, 2018 and $47,099 at December 31, 2017) Loans and Leases Receivable, Net Amount Federal Home Loan Bank of Boston stock Federal Home Loan Bank Stock Accrued interest receivable Interest Receivable Deferred tax asset, net Deferred Tax Assets, Net Premises and equipment, net Property, Plant and Equipment, Net Goodwill Goodwill Core deposit intangible Finite-Lived Core Deposits, Gross Cash surrender value of bank-owned life insurance Bank Owned Life Insurance Other assets Other Assets Total assets Assets LIABILITIES AND STOCKHOLDERS’ EQUITY Liabilities and Equity [Abstract] Liabilities: Liabilities [Abstract] Deposits: Deposits [Abstract] Non-interest-bearing Noninterest-bearing Deposit Liabilities Interest-bearing Interest-bearing Deposit Liabilities Total deposits Deposits Mortgagors’ and investors’ escrow accounts Mortgagors And Investors Escrow Accounts Mortgagor's and investors' escrow accounts. Advances from the Federal Home Loan Bank Accrued expenses and other liabilities Accrued Expenses And Other Liabilities Accrued expenses and other liabilities. Total liabilities Liabilities Stockholders’ equity: Stockholders' Equity Attributable to Parent [Abstract] Preferred stock (no par value; 2,000,000 authorized; no shares issued) Preferred Stock, Value, Issued Common stock (no par value; authorized 120,000,000 shares; 51,146,888 and 51,044,752 shares issued and outstanding at September 30, 2018 and December 31, 2017, respectively) Common Stock, Value, Issued Additional paid-in capital Additional Paid in Capital, Common Stock Unearned compensation - ESOP Unearned ESOP Shares Retained earnings Retained Earnings (Accumulated Deficit) Accumulated other comprehensive loss, net of tax Accumulated Other Comprehensive Income (Loss), Net of Tax Total stockholders’ equity Stockholders' Equity Attributable to Parent Total liabilities and stockholders’ equity Liabilities and Equity Schedule of Assets Sold under Agreements to Repurchase [Table] Schedule of Assets Sold under Agreements to Repurchase [Table] U.S. Agency Securities Securities or Other Assets Sold under Agreements to Repurchase, Maturity Periods [Axis] Repurchase Agreements and Similar Transactions, Maturity Periods [Axis] Assets Sold under Agreements to Repurchase, Maturity Period [Domain] Repurchase Agreements and Similar Transactions, Maturity Periods [Domain] Maturity Overnight Maturity Overnight [Member] Maturity Up To 1 Year Maturity Up To 1 Year [Member] Maturity Up To 1 Year [Member] Maturity 1 to 3 Years Maturity 1 to 3 Years [Member] Maturity 1 to 3 Years [Member] Maturity Greater Than 3 Years Maturity Greater Than 3 Years [Member] Maturity Greater Than 3 Years [Member] Assets Sold under Agreements to Repurchase [Line Items] Assets Sold under Agreements to Repurchase [Line Items] Borrowings under repurchase agreements Securities Sold under Agreements to Repurchase, Gross Including Not Subject to Master Netting Arrangement Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Asset Class [Axis] Asset Class [Axis] Asset Class [Domain] Asset Class [Domain] Available for Sale Securities Available-for-sale Securities [Member] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Balance at beginning of period Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value Sales Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales Principal payments and net accretion Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Assets Principal Payments Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Assets Principal Payments Issuances Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances Total realized gains (losses) included in earnings / Change in fair value recognized in net income Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings Total unrealized gains (losses) included in other comprehensive income/loss Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) Balance at end of period Measurement Input Type [Axis] Measurement Input Type [Axis] Measurement Input Type [Domain] Measurement Input Type [Domain] Discount Rate Measurement Input, Discount Rate [Member] Cumulative Default % Measurement Input, Loss Severity [Member] Loss Given Default Measurement Input, Default Rate [Member] Cost to Service Measurement Input, Cost to Sell [Member] Float Earnings Rate Measurement Input, Floating Earnings Rate [Member] Measurement Input, Floating Earnings Rate [Member] Minimum Minimum [Member] Fair Value Measurement Inputs and Valuation Techniques [Line Items] Fair Value Measurement Inputs and Valuation Techniques [Line Items] Fair Value Fair Value, Mortgage Servicing Rights Servicing Asset Unobservable Input, Asset-backed securities Debt Securities, Available-for-sale, Measurement Input Unobservable Input, Mortgage Servicing Rights Servicing Asset, Measurement Input Financing Receivable, Impaired [Line Items] Financing Receivable, Impaired [Line Items] Average Recorded Investment Impaired Financing Receivable, Average Recorded Investment Interest Income Recognized Impaired Financing Receivable, Interest Income, Accrual Method Schedule of Financing Receivables Past Due [Table] Schedule of Financing Receivables Past Due [Table] Financing Receivables, Period Past Due [Axis] Financing Receivables, Period Past Due [Axis] Financing Receivables, Period Past Due [Domain] Financing Receivables, Period Past Due [Domain] 30-59 Days Past Due Financing Receivables, 30 to 59 Days Past Due [Member] 60-89 Days Past Due Financing Receivables, 60 to 89 Days Past Due [Member] Past Due 90 Days or More Financing Receivables, Equal to Greater than 90 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 Past Due 90 Days or More and Still Accruing Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing Loans on Non-accrual Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Accumulated Other Comprehensive Income (Loss) [Table] Accumulated Other Comprehensive Income (Loss) [Table] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Benefit plans Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] Securities available-for-sale Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] Interest rate swaps Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member] AOCI Attributable to Parent AOCI Attributable to Parent [Member] Retained Earnings Retained Earnings [Member] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Net unrealized gain (loss) AOCI before Tax, Attributable to Parent Tax effect AOCI Tax, Attributable to Parent Adoption of ASU No. 2018-02 (see Note 10) Tax Cuts And Jobs Act, Reclassification From AOCI To Retained Earnings, Tax Effect Tax Cuts And Jobs Act, Reclassification From AOCI To Retained Earnings, Tax Effect Impaired loans Impaired Loans Fair Value Disclosure Losses Included In Earnings Impaired loans fair value disclosure losses included in earnings. Other real estate owned Other Real Estate Owned Fair Value Disclosure Gains (Losses) Included In Earnings Other Real Estate Owned Fair Value Disclosure Gains Losses Included In Earnings Total Assets Fair Value Disclosure Losses Included In Earnings Assets fair value disclosure losses included in earnings. Allowance for loan losses, loans receivable Preferred stock, shares authorized (in shares) Preferred Stock, Shares Authorized Preferred stock, shares issued (in shares) Preferred Stock, Shares Issued Common stock, shares authorized (in shares) Common Stock, Shares Authorized Common stock, shares issued (in shares) Common Stock, Shares, Issued Common stock, shares outstanding (in shares) Common Stock, Shares, Outstanding Statement of Stockholders' Equity [Abstract] Statement [Table] Statement [Table] Common Stock Common Stock [Member] Additional Paid-in Capital Additional Paid-in Capital [Member] Unearned Compensation - ESOP Deferred Compensation, Share-based Payments [Member] Accumulated Other Comprehensive Loss Statement [Line Items] Statement [Line Items] Increase (Decrease) in Stockholders' Equity Increase (Decrease) in Stockholders' Equity [Roll Forward] Balance (in shares) Balance Adoption of ASU No. 2016-01 (see Note 3) Cumulative Effect of New Accounting Principle in Period of Adoption Comprehensive income Comprehensive Income (Loss), Net of Tax, Attributable to Parent Common stock repurchased (in shares) Stock Repurchased During Period, Shares Common stock repurchased Stock Repurchased During Period, Value Stock-based compensation expense Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition ESOP shares released or committed to be released Amortization of ESOP Award Shares issued for stock options exercised and SARs (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Shares issued for stock options exercised Stock Issued During Period, Value, Stock Options Exercised Shares issued for restricted stock grants (in shares) Stock Issued During Period, Shares, Restricted Stock Award, Gross Shares issued for restricted stock grants Stock Issued During Period, Value, Restricted Stock Award, Gross Shares cancelled for restricted stock forfeitures (in shares) Stock Issued During Period, Shares, Restricted Stock Award, Forfeited Shares cancelled for restricted stock forfeitures Stock Issued During Period, Value, Restricted Stock Award, Forfeitures Cancellation of shares for tax withholding (in shares) Shares Paid for Tax Withholding for Share Based Compensation Cancellation of shares for tax withholding Adjustments Related to Tax Withholding for Share-based Compensation Dividends paid ($0.24 and $0.36 per common share) Dividends, Common Stock, Cash Balance (in shares) Balance Net Income Per Share Earnings Per Share [Text Block] Schedule of Financing Receivable, Recorded Investment, Credit Quality Indicator [Table] Schedule of Financing Receivable, Recorded Investment, Credit Quality Indicator [Table] External Credit Rating by Grouping [Axis] Internal Credit Assessment [Axis] External Credit Rating by Grouping [Domain] Internal Credit Assessment [Domain] Loans rated 1-5 Pass [Member] Loans rated 6 Special Mention [Member] Loans rated 7 Substandard [Member] Loans rated 8 Doubtful [Member] Loans rated 9 Unlikely to be Collected Financing Receivable [Member] Financing Receivable, Recorded Investment [Line Items] Financing Receivable, Recorded Investment [Line Items] Total loans Goodwill and Intangible Assets Disclosure [Abstract] Goodwill Schedule of Finite-Lived Intangible Assets [Table] Schedule of Finite-Lived Intangible Assets [Table] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets, Major Class Name [Domain] Finite-Lived Intangible Assets, Major Class Name [Domain] Core deposits Core Deposits [Member] Finite-Lived Intangible Assets [Line Items] Finite-Lived Intangible Assets [Line Items] Core Deposit Intangibles Finite-lived Intangible Assets [Roll Forward] Balance at Beginning of Period Finite-Lived Intangible Assets, Net Amortization expense Balance at End of Period Regulatory Matters Regulatory Capital Requirements under Banking Regulations [Text Block] Statement of Comprehensive Income [Abstract] Income tax benefit (expense) associated with the reclassification adjustment realized in net income for sale of securities Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax Income tax benefit associated with the reclassification adjustment for expense realized in net income for interest rate swaps Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, Tax Income tax benefit associated with the reclassification adjustment for losses recognized in net periodic benefit cost Other Comprehensive Income Amortization Of Post Retirement Benefit Plan Net Prior Service Cost Recognized In Net Periodic Pension Cost Tax Other Comprehensive Income Amortization Of Post Retirement Benefit Plan Net Prior Service Cost Recognized In Net Periodic Pension Cost Tax Borrowings Federal Home Loan Bank Advances And Stock And Other Borrowings Disclosure [Text Block] Federal Home Loan Bank Advances And Stock And Other Borrowings Disclosure [Text Block] Fair Value Measurements Fair Value Disclosures [Text Block] Income Statement Location [Axis] Income Statement Location [Axis] Income Statement Location [Domain] Income Statement Location [Domain] Net Gain From Sales of Loans Net Gain From Sales of Loans [Member] Net Gain From Sales of Loans [Member] Mortgage loans held for sale Gains (Losses) In Fair Value, Mortgage Loans Held For Sale Gains (Losses) In Fair Value, Mortgage Loans Held For Sale Schedule of Basic and Diluted Net Income Per Share Schedule of Weighted Average Number of Shares [Table Text Block] Schedule of Financing Receivable, Allowance for Credit Losses [Table] Schedule of Financing Receivable, Allowance for Credit Losses [Table] Activity in the Allowance for Loan Losses Allowance for Loan and Lease Losses [Roll Forward] Balance, beginning of period Provision (credit) for loan losses Loans charged off Allowance for Loan and Lease Losses, Write-offs Recoveries of loans previously charged off Allowance for Loan and Lease Loss, Recovery of Bad Debts Balance, end of period Net income Other comprehensive (loss) income: Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent [Abstract] Securities available-for-sale: Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax, Portion Attributable to Parent [Abstract] Unrealized holding (losses) gains Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, before Tax Reclassification adjustment for losses (gains) realized in operations Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax Net unrealized (losses) gains Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, before Tax Tax effect - benefit (expense) Other Comprehensive Income (Loss), Available-for-sale Securities, Tax, Portion Attributable to Parent Net-of-tax amount - securities available-for-sale Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, after Tax Interest rate swaps designated as cash flow hedges: Interest Rate Cash Flow Hedges [Abstract] Unrealized gains (losses) Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), before Reclassification and Tax Reclassification adjustment for losses recognized in interest expense Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax Net unrealized gains (losses) Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification, before Tax Tax effect - (expense) benefit Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification, Tax Net-of-tax amount - interest rate swaps Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax Pension and Post-retirement plans: Other Comprehensive Income Post Retirement Plans Adjustment Net Of Tax Portion Attributable To Parent [Abstract] Other Comprehensive Income Post Retirement Plans Adjustment Net of Tax Portion Attributable to Parent [Abstract] Reclassification adjustment for prior service costs recognized in net periodic benefit cost Other Comprehensive Income Reclassification Of Post Retirement Plan Net Transition Asset Obligation Recognized In Net Periodic Benefit Cost Before Tax Reclassification adjustment for prior service costs recognized in net periodic benefit cost. Reclassification adjustment for losses recognized in net periodic benefit cost Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, before Tax Net change in gains and prior service costs Other Comprehensive Income Reclassification Of Post Retirement Plans Net Gain Loss Recognized In Net Periodic Benefit Cost Before Tax Change in gains or losses and prior service costs or credits. Tax effect - expense Other Comprehensive Income Loss Reclassification Adjustment From Aoci Postretirement Benefit Plans For Net Transition Asset Obligation Tax Tax effect. Net-of-tax amount - pension and post-retirement plans Other Comprehensive Income Loss Defined Benefit Plans And Post Retirement Plans Adjustment Net Of Tax Portion Attributable To Parent Other Comprehensive Income Loss Defined Benefit Plans And Post Retirement Plans Adjustment Net Of Tax Portion Attributable To Parent Total other comprehensive (loss) income Other Comprehensive Income (Loss), Net of Tax Comprehensive income Derivative Instruments and Hedging Activities Disclosure [Abstract] Schedule of Interest Rate Swap Agreements and Non-Hedging Derivative Assets and Liabilities Interest Rate Swap Agreements And Non Hedging Derivative Assets And Liabilities [Table Text Block] Interest Rate Swap Agreements And Non Hedging Derivative Assets And Liabilities [Table Text Block] Tabular Disclosure of Fair Values of Derivative Instruments Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block] Schedule of Effect of Derivative Instruments in Statements of Operations Derivative Instruments, Gain (Loss) [Table Text Block] Other Commitments [Table] Other Commitments [Table] Other Commitments [Axis] Other Commitments [Axis] Other Commitments [Domain] Other Commitments [Domain] Tax credit partnership Investment, Tax Credit Partnership Associated With Alternative Energy [Member] Investment, Tax Credit Partnership Associated With Alternative Energy [Member] Other Commitments [Line Items] Other Commitments [Line Items] Net carrying balance of investments Other Commitment Capital contribution commitments Contractual Obligation Derivative [Table] Derivative [Table] Derivative Instrument [Axis] Derivative Instrument [Axis] Derivative Contract [Domain] Derivative Contract [Domain] Interest rate contract Interest Rate Contract [Member] Borrower Borrower [Member] Borrower [Member] Brokerage activities Brokerage Activities [Member] Brokerage Activities [Member] Interest Rate Swap, Risk Participation Agreement Interest Rate Swap, Risk Participation Agreement [Member] Interest Rate Swap, Risk Participation Agreement [Member] Interest rate swap, risk participation agreement, credit enhancements provided by counterparty Interest Rate Swap, Risk Participation Agreement, Credit Enhancements Provided By Counterparty [Member] Interest Rate Swap, Risk Participation Agreement, Credit Enhancements Provided By Counterparty [Member] Interest rate swap, risk participation agreement, credit enhancements provided by the bank Interest Rate Swap, Risk Participation Agreement, Credit Enhancements Provided By The Bank [Member] Interest Rate Swap, Risk Participation Agreement, Credit Enhancements Provided By The Bank [Member] Forward starting loan level swaps - borrower Forward Starting Loan Level Swaps - Borrower [Member] Forward Starting Loan Level Swaps - Borrower [Member] Forward starting loan level swaps - dealer Forward Starting Loan Level Swaps - Dealer [Member] Forward Starting Loan Level Swaps - Dealer [Member] Hedging Designation [Axis] Hedging Designation [Axis] Hedging Designation [Domain] Hedging Designation [Domain] Derivatives designated as hedging instruments Designated as Hedging Instrument [Member] Derivatives not designated as hedging instruments Not Designated as Hedging Instrument [Member] Hedging Relationship [Axis] Hedging Relationship [Axis] Hedging Relationship [Domain] Hedging Relationship [Domain] Cash flow hedges Cash Flow Hedging [Member] Fair value hedges Fair Value Hedging [Member] Derivative [Line Items] Derivative [Line Items] Estimated reduction/increase to interest expense Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net Forecasted transactions, period Maximum Length of Time Hedged in Interest Rate Cash Flow Hedge Number of derivative instruments Derivative, Number of Instruments Held Derivative notional amount Derivative, Notional Amount Number of counterparties in risk participation agreements Derivative, Number Of Counterparties Derivative, Number Of Counterparties Number of borrowers involved in the risk participation agreements Number Of Customers Number Of Customers Counterparty participation level, percentage Counterparty Participation Level, Percent Counterparty Participation Level, Percent Fair value of net derivatives, liability position Derivative, Net Liability Position, Aggregate Fair Value Amount of collateral Collateral Already Posted, Aggregate Fair Value Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Subordinated debentures Other Secured Debt And Capital Lease Obligations [Member] Secured Debt And Capital Lease Obligations [Member] Wholesale repurchase agreements Customer repurchase agreements Debt Instrument [Line Items] Debt Instrument [Line Items] Derivatives and Hedging Activities Derivative Instruments and Hedging Activities Disclosure [Text Block] Schedule of Financing Receivable, Troubled Debt Restructurings [Table] Schedule of Financing Receivable, Troubled Debt Restructurings [Table] Financing Receivable, Modifications [Line Items] Financing Receivable, Modifications [Line Items] Number of Loans Financing Receivable, Modifications, Subsequent Default, Number of Contracts Recorded Investment Financing Receivable, Modifications, Subsequent Default, Recorded Investment Goodwill and Core Deposit Intangibles Goodwill and Intangible Assets Disclosure [Text Block] Stock Appreciation Rights (SARs) Stock Appreciation Rights (SARs) [Member] Total unrecognized compensation cost related to outstanding stock options Unrecognized cost related to outstanding stock options, period of recognition Stock options granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross SAR options exercised (in shares) New Accounting Pronouncements or Change in Accounting Principle [Table] New Accounting Pronouncements or Change in Accounting Principle [Table] Scenario [Axis] Scenario [Axis] Scenario, Unspecified [Domain] Scenario, Unspecified [Domain] Pro Forma Pro Forma [Member] Forecast Scenario, Forecast [Member] Adjustments for New Accounting Pronouncements [Axis] Adjustments for New Accounting Pronouncements [Axis] Type of Adoption [Domain] Type of Adoption [Domain] Accounting Standards Update 2017-08 Accounting Standards Update 2017-08 [Member] Accounting Standards Update 2016-02 Accounting Standards Update 2016-02 [Member] New Accounting Pronouncements or Change in Accounting Principle [Line Items] New Accounting Pronouncements or Change in Accounting Principle [Line Items] Impact on retained earnings New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets Increase to assets Increase to liabilities Recorded Investment, Impaired loans without a valuation allowance Impaired Financing Receivable, with No Related Allowance, Recorded Investment Unpaid Principal Balance, Impaired loans without a valuation allowance Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance Recorded Investment, Impaired loans with a valuation allowance Impaired Financing Receivable, with Related Allowance, Recorded Investment Unpaid Principal Balance, Impaired loans with a valuation allowance Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance Recorded Investment, Total impaired loans Unpaid Principal Balance, Total impaired loans Impaired Financing Receivable, Unpaid Principal Balance Statement of Cash Flows [Abstract] Cash flows from operating activities: Net Cash Provided by (Used in) Operating Activities [Abstract] Adjustments to reconcile net income to net cash provided by (used in) operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Amortization of premiums and discounts on investments, net Accretion (Amortization) of Discounts and Premiums, Investments Amortization of intangible assets and purchase accounting marks, net Accretion of Intangible Assets and Purchase Accounting Marks Accretion of Intangible Assets and Purchase Accounting Marks Amortization of subordinated debt issuance costs Amortization of Debt Issuance Costs ESOP expense Employee Stock Ownership Plan (ESOP), Compensation Expense Gains on sales of securities, net Net unrealized gain on marketable equity securities Marketable Securities, Gain (Loss) Loans originated for sale Payments for Origination and Purchases of Loans Held-for-sale Principal balance of loans sold Proceeds from Sale of Loans Held-for-sale Increase in mortgage servicing asset Servicing Asset at Fair Value, Period Increase (Decrease) Gain on sales of other real estate owned Gains (Losses) on Sales of Other Real Estate Net change in mortgage banking fair value adjustments Change In Mortgage Banking Fair Value Adjustment Change in Mortgage Banking Fair Value Adjustment (Gain) loss on disposal of equipment Gain (Loss) on Disposition of Property Plant Equipment Write-downs of other real estate owned Impairment of Real Estate Depreciation and amortization of premises and equipment Depreciation, Depletion and Amortization, Nonproduction Net loss on limited partnership investments Gain (Loss) on Limited Partnerships Gain (Loss) on Limited Partnerships Deferred income tax (benefit) expense Deferred Income Tax Expense (Benefit) Increase in cash surrender value of bank-owned life insurance Bank Owned Life Insurance Income Income recognized from death benefits on bank-owned life insurance Bank Owned Life Insurance Income From Death Benefit Bank Owned Life Insurance Income From Death Benefit Net change in: Increase (Decrease) in Operating Capital [Abstract] Deferred loan fees and premiums Increase (Decrease) in Loans, Deferred Income Accrued interest receivable Increase (Decrease) in Accrued Interest Receivable, Net Other assets Increase (Decrease) in Other Operating Assets Accrued expenses and other liabilities Increase (Decrease) in Other Accounts Payable and Accrued Liabilities Net cash provided by operating activities Net Cash Provided by (Used in) Operating Activities Cash flows from investing activities: Net Cash Provided by (Used in) Investing Activities [Abstract] Proceeds from sales of available-for-sale securities Proceeds from Sale of Available-for-sale Securities Proceeds from calls and maturities of available-for-sale securities Proceeds from Maturities, Prepayments and Calls of Debt Securities, Available-for-sale Principal payments on available-for-sale securities Proceeds from Sale and Maturity of Debt Securities, Available-for-sale Principal payments on held-to-maturity securities Proceeds from Maturities, Prepayments and Calls of Held-to-maturity Securities Purchases of available-for-sale securities Payments to Acquire Debt Securities, Available-for-sale Redemption of FHLBB and other restricted stock Payments for (Proceeds from) Federal Home Loan Bank Stock Purchase of FHLBB stock Payments to Acquire Federal Home Loan Bank Stock Proceeds from sale of other real estate owned Proceeds from Sale of Other Real Estate Purchases of loans Payments to Purchase Loans Held-for-sale Loan originations, net of principal repayments Payments For Loan Originations Net Of Principal Payments Loan originations, net of principal payments. Proceeds from bank-owned life insurance death benefits Proceeds from Life Insurance Policy Purchases of bank-owned life insurance Payment to Acquire Life Insurance Policy, Investing Activities Proceeds from redemption of bank-owned life insurance Proceeds From Surrender Of Bank-Owned Life Insurance Proceeds From Surrender Of Bank-Owned Life Insurance Purchases of premises and equipment Payments to Acquire Property, Plant, and Equipment Proceeds from sales of equipment Proceeds from Sale of Machinery and Equipment Net cash used in investing activities Net Cash Provided by (Used in) Investing Activities Cash flows from financing activities: Net Cash Provided by (Used in) Financing Activities [Abstract] Net increase (decrease) in non-interest-bearing deposits Net Change Noninterest Bearing Deposits Net increase in non-interest-bearing deposits. Net increase in interest-bearing deposits Net Change Interest Bearing Deposits Net increase in interest-bearing deposits. Net increase (decrease) in mortgagors’ and investors’ escrow accounts Increase (Decrease) In Deposit Other Assets, Financing Activities Increase (Decrease) In Deposit Other Assets, Financing Activities Net decrease in short-term FHLBB advances Increase (Decrease) in Loans from Federal Home Loan Banks Repayments of long-term FHLBB advances Payments for FHLBank Advance, Investing Activities Repayments of called FHLBB advances Payments For Called Federal Home Loan Bank Advances Payments For Called Federal Home Loan Bank Advances Proceeds from long-term FHLBB advances Proceeds from FHLBank Advance, Investing Activities Net change in other borrowings Increase Decrease In Repurchase Agreements Increase Decrease In Repurchase Agreements Proceeds from exercise of stock options Proceeds From Exercise Of Stock Options And Stock Purchase Plan Proceeds from exercise of stock options and stock purchase plan. Common stock repurchased Payments For Purchase Of Treasury Stock Common stock repurchased. Cancellation of shares for tax withholding Payments Related to Tax Withholding for Share-based Compensation Cash dividend paid on common stock Payments of Dividends Net cash provided by financing activities Net Cash Provided by (Used in) Financing Activities Net increase (decrease) in cash and cash equivalents Cash and Cash Equivalents, Period Increase (Decrease) Cash and cash equivalents, beginning of period Cash and cash equivalents, end of period Supplemental Disclosures of Cash Flow Information: Supplemental Cash Flow Elements [Abstract] Cash paid during the year for: Supplemental Disclosures Of Cash Flow Information Cash Paid During Period [Abstract] Cash paid during the year for: Interest Interest Paid, Excluding Capitalized Interest, Operating Activities Income taxes, net Income Taxes Paid, Net Transfer of loans to other real estate owned Real Estate Owned, Transfer to Real Estate Owned Change in due to broker, investment purchases Increase (Decrease) in Payables to Broker-Dealers and Clearing Organizations Employee Stock Ownership Plan 2011 Employee Stock Ownership Plan Two Zero One One [Member] Employee Stock Ownership Plan Two Zero One One [Member] Borrowings from the company Origination of Loans to Employee Stock Ownership Plans Purchase of common stock (in shares) Employee Stock Ownership Plan Number Of Shares Purchased Employee stock ownership plan number of shares purchased. Employee stock ownership plan loan outstanding balance Employee Stock Ownership Plan Loan Outstanding Balance Employee stock ownership plan loan outstanding balance. Bank's discretionary contributions to the ESOP over a remaining period Employee Stock Ownership Plan Esop Period Remaining To Repay Outstanding Loan Amount Employee Stock Ownership Plan ESOP Period Remaining To Repay Outstanding Loan Amount Interest rate for the ESOP loans Employee Stock Ownership Plan Basis Spread On Variable Rate Employee stock ownership plan basis spread on variable rate. ESOP compensation expense Document And Entity Information [Abstract] Document and entity information. 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 Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Entity Filer Category Entity Filer Category Entity Emerging Growth Company Entity Emerging Growth Company Entity Small Business Entity Small Business Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Changes in the Carrying Amount of Goodwill and Core Deposit Intangible Assets Schedule of Intangible Assets and Goodwill [Table Text Block] Aggregate principal amount of residential real estate mortgage loans held for sale Loans Receivable Held-for-sale, Net, Not Part of Disposal Group, Mortgage Aggregate fair value of mortgage loans held for sale Residential real estate mortgage loans held for sale 90 days or more past due Residential Real Estate Mortgage Loans Held For Sale 90 Days Or More Past Due Residential Real Estate Mortgage Loans Held For Sale 90 Days Or More Past Due Transfers in and out of Level 1, Level 2 and Level 3 measurements Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net Fixed rate mortgage loans term Fixed Rate Mortgage Loans Term Fixed rate mortgage loans term. Maturity period, Term 1 Fixed Rate Mortgage Loans Term One Fixed rate mortgage loans term one. Maturity period, Term 2 Fixed Rate Mortgage Loans Term Two Fixed rate mortgage loans term two. Maturity period, Term 3 Fixed Rate Mortgage Loans Term Three Fixed rate mortgage loans term three. Estimated fallout rate based upon historical averages Estimate Fallout Rate Based Upon Historical Average Estimate fallout rate based upon historical average. Liabilities measured at fair value on a non-recurring basis Financial and Nonfinancial Liabilities, Fair Value Disclosure Core deposits estimated useful life Finite-Lived Intangible Asset, Useful Life Financial Instruments Contract Amounts Represent Credit Risk Schedule of Fair Value, Off-balance Sheet Risks [Table Text Block] Investor non-owner occupied Construction Commercial Construction Loans [Member] Commercial Construction Loans [Member] Residential construction Residential Construction Loans [Member] Residential Construction Loans [Member] Concentration Risk Type [Axis] Concentration Risk Type [Axis] Concentration Risk Type [Domain] Concentration Risk Type [Domain] Loans Receivable Concentration Risk Loans Receivable Concentration Risk [Member] Loans Receivable Concentration Risk [Member] Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Domain] Loans Receivable Loans Receivable [Member] Total loans Percent of total loans Concentration Risk, Percentage Net deferred loan costs and premiums Unamortized Loan Commitment and Origination Fees and Unamortized Discounts or Premiums Allowance for loan losses Loans - net Number of Stock Options Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] Outstanding at December 31, 2017 (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number Granted (in shares) Exercised (in shares) Forfeited or expired (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures in Period Outstanding at September 30, 2018 (in shares) Stock options vested and exercisable at September 30, 2018 (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Number Stock Options, Weighted- Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract] Outstanding at December 31, 2017 (in usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price Granted (in usd per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price Exercised (in usd per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price Forfeited or expired (in usd per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Forfeitures in Period, Weighted Average Exercise Price Outstanding at September 30, 2018 (in usd per share) Stock options vested and exercisable at September 30, 2018 (in usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Weighted Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] Weighted-Average Remaining Contractual Term, Outstanding at September 30, 2018 Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term Weighted-Average Remaining Contractual Term, Stock options vested and exercisable at September 30, 2018 Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term Aggregate Intrinsic Value, Outstanding at September 30, 2018 Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value Aggregate Intrinsic Value, Stock options vested and exercisable at September 30, 2018 Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value Allocated shares (in shares) Employee Stock Ownership Plan (ESOP), Number of Allocated Shares Shares allocated for release (in shares) Employee Stock Ownership Plan (ESOP), Number of Committed-to-be-Released Shares Unreleased shares (in shares) Employee Stock Ownership Plan (ESOP), Number of Suspense Shares Total ESOP shares (in shares) Employee Stock Ownership Plan (ESOP), Shares in ESOP Market value of unreleased shares Employee Stock Ownership Plan (ESOP), Fair Value of Shares Subject to Repurchase Obligation Schedule Of Available For Sale And Held To Maturity Securities [Table] Schedule Of Available For Sale And Held To Maturity Securities [Table] Schedule of available for sale and held to maturity securities. Investment Type [Axis] Investment Type [Axis] Investments [Domain] Investments [Domain] Industrial Industrial [Member] Industrial. Oil and gas Oil and Gas Properties [Member] Schedule Of Available For Sale And Held To Maturity Securities [Line Items] Schedule Of Available For Sale And Held To Maturity Securities [Line Items] Schedule of available for sale and held to maturity securities. Available-for-sale Securities, Debt Securities Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] Amortized Cost Debt Securities, Available-for-sale, Amortized Cost Gross Unrealized Gains Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax Gross Unrealized Losses Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax Available-for-sale, Marketable Equity Securities Available For Sale Equity Securities, Fair Value To Amortized Cost Basis [Abstract] Available For Sale Equity Securities, Amortized Cost Basis [Abstract] Amortized Cost Available-for-sale Equity Securities, Amortized Cost Basis Gross Unrealized Gains Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax Gross Unrealized Losses Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax Fair Value Available-for-sale, Securities Available For Sale Securities, Total, Amortized Cost Basis [Abstract] Available For Sale Securities, Total, Amortized Cost Basis [Abstract] Amortized Cost Available-for-sale Securities, Amortized Cost Basis 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 Held-to-maturity, Debt Securities Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract] Amortized Cost Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment Gross Unrealized Gains Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain Gross Unrealized Losses Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss Fair Value Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] Forward loan sale commitments Forward Loan Sale Commitments [Member] Forward Loan Sale Commitments [Member] Derivative loan commitments Derivative Loan Commitments [Member] Derivative Loan Commitments [Member] Forward starting loan level swap Forward Starting Loan Level Swap [Member] Forward Starting Loan Level Swap [Member] Counterparty Name [Axis] Counterparty Name [Axis] Counterparty Name [Domain] Counterparty Name [Domain] With customers With Customers [Member] With Customers [Member] With counterparties With Counterparties [Member] With Counterparties [Member] Balance Sheet Location [Axis] Balance Sheet Location [Axis] Balance Sheet Location [Domain] Balance Sheet Location [Domain] Other Assets Other Assets [Member] Other Liabilities Other Liabilities [Member] Derivatives, Fair Value [Line Items] Derivatives, Fair Value [Line Items] Derivative Assets Derivative Asset, Fair Value, Gross Asset Derivative Liabilities Derivative Liability, Fair Value, Gross Liability ASU 2016-01 Accounting Standards Update 2016-01 [Member] Net unrealized gain (loss) on securities available for sale Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain (Loss), before Tax Net unrealized gain (loss) on securities available for sale, income taxes (benefit) Accumulated Other Comprehensive Income (Loss), Available-For-Sale Securities Adjustment, Tax Accumulated Other Comprehensive Income (Loss), Available-For-Sale Securities Adjustment, Tax Accumulated other comprehensive income (loss) Accumulated Other Comprehensive Income (Loss), Debt Securities, Available-for-sale, Adjustment, after Tax Increase to retained earnings Decrease to accumulated other comprehensive income Unrealized gains, marketable equity securities Equity Securities, FV-NI, Unrealized Gain (Loss) Fair value, marketable equity securities Gross unrealized gains, marketable equity securities Equity Securities, FV-NI, Gross Unrealized Gain Equity Securities, FV-NI, Gross Unrealized Gain Encumbered securities pledged as derivative collateral Pledged Assets Separately Reported, Securities Pledged as Collateral, at Fair Value Gross gains realized on the sale of available-for-sale securities Debt Securities, Available-for-sale, Realized Gain Gross gains realized on the sale of available-for-sale securities Available-for-sale Securities, Gross Realized Gains Gross losses realized on the sale of available-for-sale securities Debt Securities, Available-for-sale, Realized Loss Gross losses realized on the sale of available-for-sale securities Available-for-sale Securities, Gross Realized Losses Estimated fair value of obligations of states and political subdivisions Estimated Fair Value Of Obligations Of States And Political Subdivisions Estimated Fair Value Of Obligations Of States And Political Subdivisions General obligation bonds General Obligation Bonds General obligation bonds. Obligations of political subdivisions