Disclosure of Compensation Related Costs, Share-based Payments [Abstract] 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 Banking and Thrift [Abstract] 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 Disclosures [Abstract] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Recurring Fair Value, Measurements, Recurring [Member] Asset Class [Axis] Asset Class [Axis] Asset Class [Domain] Asset Class [Domain] Available for Sale Securities Available-for-sale Securities [Member] Major Types of Debt and Equity Securities [Axis] Major Types of Debt and Equity Securities [Axis] Major Types of Debt and Equity Securities [Domain] Major Types of Debt and Equity Securities [Domain] Mortgage servicing rights Mortgage Loan Servicing Rights [Member] Mortgage Loan Servicing Rights [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 Receivables [Abstract] Schedule of Accounts, Notes, Loans and Financing Receivable [Table] Schedule of Accounts, Notes, Loans and Financing Receivable [Table] Class of Financing Receivable, Type [Axis] Class of Financing Receivable, Type [Axis] Class of Financing Receivable, Type [Domain] Class of Financing Receivable, Type [Domain] 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 Accounting Policies [Abstract] Summary of Significant Accounting Policies Organization, Consolidation and Presentation of Financial Statements Disclosure and Significant Accounting Policies [Text Block] 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] Investments, Debt and Equity Securities [Abstract] 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. 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] 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. Maturity, Available-for-sale Securities, Amortized Cost: Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity [Abstract] Within 1 year Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis After 1 year through 5 years Available-for-sale Securities, Debt Maturities, Year Two Through Five, Amortized Cost Basis After 5 years through 10 years Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Amortized Cost Basis After 10 years Available-for-sale Securities, Debt Maturities, after Ten Years, Amortized Cost Basis Single maturity date Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis Without single maturity date Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis Amortized Cost Available-for-sale Debt Securities, Amortized Cost Basis Maturity, Available-for-sale Securities, Fair Value: Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract] Within 1 year Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value After 1 year through 5 years Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value After 5 years through 10 years Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value After 10 years Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value Single maturity date Available-for-sale Securities, Debt Maturities, Single Maturity Date Without single maturity date Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Fair Value Fair Value Available-for-sale Securities, Debt Securities 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 [Axis] Fair Value, Hierarchy [Axis] Fair Value Hierarchy [Domain] Fair Value Hierarchy [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 Available-for-sale securities Available-for-sale Securities Held-to-maturity securities Held-to-maturity Securities, 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 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 Stock-Based Compensation Plans Disclosure of Compensation Related Costs, Share-based Payments [Text Block] Mortgage servicing rights: Servicing Asset at Fair Value, Amount [Roll Forward] Balance at beginning of period Servicing Asset at Fair Value, Amount 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 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 Inputs, Assets, Quantitative Information [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] 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 Liabilities, Fair Value Disclosure, Nonrecurring Schedule of Gain (Loss) on Investments [Table] Schedule of Gain (Loss) on Investments [Table] Corporate debt securities Corporate Debt Securities [Member] Obligations of states and political subdivisions US States and Political Subdivisions Debt Securities [Member] Gain (Loss) on Investments [Line Items] Gain (Loss) on Investments [Line Items] Available-for-sale Securities, Fair Value: Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract] Less Than 12 Months Debt Securities, Available-For-Sale, Continuous Unrealized Loss Position Less Than 12 Months Debt Securities, Available-For-Sale, Continuous Unrealized Loss Position Less Than 12 Months 12 Months or More Debt Securities, Available-For-Sale, Continuous Unrealized Loss Position, 12 Months Or Longer Debt Securities, Available-For-Sale, Continuous Unrealized Loss Position, 12 Months Or Longer Total Debt Securities, Available-For-Sale, Unrealized Loss Position Debt Securities, Available-For-Sale, Unrealized Loss Position Available-for-sale Securities, Unrealized Loss: Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract] Less Than 12 Months Debt Securities, Available-For-Sale, Continuous Unrealized Loss Position, Less Than 12 Months, Accumulated Loss Debt Securities, Available-For-Sale, Continuous Unrealized Loss Position, Less Than 12 Months, Accumulated Loss 12 Months or More Debt Securities, Available-For-Sale, Continuous Unrealized Loss Position, 12 Months Or Longer, Accumulated Loss Debt Securities, Available-For-Sale, Continuous Unrealized Loss Position, 12 Months Or Longer, Accumulated Loss Total Debt Securities, Available-For-Sale, Unrealized Loss Position, Accumulated Loss Debt Securities, Available-For-Sale, Unrealized Loss Position, Accumulated Loss Held-to-maturity Securities, Fair Value: Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value [Abstract] Less Than 12 Months Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value Unrealized Loss Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value Total Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value Held-to-maturity Securities, Unrealized Loss: Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract] Less Than 12 Months Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 12 Months or More Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss Total Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss Goodwill and Intangible Assets Disclosure [Abstract] Finite-Lived Intangible Assets, Amortization Expense, Maturity Schedule [Abstract] Finite-Lived Intangible Assets, Amortization Expense, Maturity Schedule [Abstract] 2018 (remaining nine months) Finite-Lived Intangible Assets, Amortization Expense, Remainder of Fiscal Year 2019 Finite-Lived Intangible Assets, Amortization Expense, Year Two 2020 Finite-Lived Intangible Assets, Amortization Expense, Year Three 2021 Finite-Lived Intangible Assets, Amortization Expense, Year Four 2022 Finite-Lived Intangible Assets, Amortization Expense, Year Five 2023 and thereafter Finite-Lived Intangible Assets, Amortization Expense, after Year Five Total remaining Finite-Lived Intangible Assets, Net Derivative Instruments and Hedging Activities Disclosure [Abstract] Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] 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 Instrument [Axis] Derivative Instrument [Axis] Derivative Contract [Domain] Derivative Contract [Domain] Interest rate swaps Interest Rate Swap [Member] 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 Derivative [Table] Derivative [Table] Forward starting interest rate swaps on future borrowings Forward Interest Rate Swap [Member] Forward Interest Rate Swap [Member] Loan level swaps - dealer Loan Level Swaps Dealer [Member] Loan Level Swaps Dealer [Member] Loan level swaps - borrowers Loan Level Rate Swaps Borrower [Member] Loan Level Rate Swaps Borrower [Member] Forward starting loan level swaps - dealer Forward Starting Loan Level Swaps - Dealer [Member] Forward Starting Loan Level Swaps - Dealer [Member] Forward starting loan level swaps - borrower Forward Starting Loan Level Swaps - Borrower [Member] Forward Starting Loan Level Swaps - Borrower [Member] Derivative [Line Items] Derivative [Line Items] Notional Amount Derivative, Notional Amount Weighted-Average Remaining Maturity (in years) Weighted Average Maturity Period Weighted Average Maturity Period Weighted-Average Interest Rate Swaps, Received Weighted Average Interest Rate Received Interest Rate Swaps Weighted Average Interest Rate Received Interest Rate Swaps Weighted-Average Interest Rate Swaps, Paid Weighted Average Interest Rate Paid Interest Rate Swaps Weighted Average Interest Rate Paid Interest Rate Swaps Estimated Fair Value, Net Asset (Liability) Derivative Assets (Liabilities), at Fair Value, Net Fair Value Inputs, Assets, Quantitative Information [Table] Fair Value Inputs, Assets, Quantitative Information [Table] Mortgage servicing rights Mortgage Loans [Member] Mortgage Loans [Member] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum Minimum [Member] Maximum Maximum [Member] Weighted Average Weighted Average [Member] Valuation Technique [Axis] Valuation Technique [Axis] Valuation Technique [Domain] Valuation Technique [Domain] Discounted Cash Flow Income Approach Valuation Technique [Member] Fair Value Inputs, Assets, Quantitative Information [Line Items] Fair Value Inputs, Assets, Quantitative Information [Line Items] Fair Value Discount Rate Fair Value Inputs, Discount Rate Cumulative Default % Fair Value Inputs, Probability of Default Loss Given Default Fair Value Inputs, Loss Given Default Fair Value Inputs, Loss Given Default Cost to Service Fair Value Inputs, Cost to Service Fair Value Inputs, Cost to Service Float Earnings Rate Fair Value Inputs, Float Earnings Rate Fair Value Inputs, Float Earnings Rate 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] New Accounting Pronouncements or Change in Accounting Principle [Table] New Accounting Pronouncements or Change in Accounting Principle [Table] 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 2017-08 [Member] Scenario [Axis] Scenario [Axis] Scenario, Unspecified [Domain] Scenario, Unspecified [Domain] Pro Forma Pro Forma [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 Future minimum lease commitments Capital Leases, Future Minimum Payments Due Schedule of Financing Receivables Past Due [Table] Schedule of Financing Receivables Past Due [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] 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 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 Schedule of Financing Receivable, Troubled Debt Restructurings [Table] Schedule of Financing Receivable, Troubled Debt Restructurings [Table] Loan Restructuring Modification [Axis] Loan Restructuring Modification [Axis] Loan Restructuring Modification [Domain] Loan Restructuring Modification [Domain] Extended Maturity Extended Maturity [Member] Adjusted Rate and Extended Maturity Contractual Interest Rate Reduction And Extended Maturity [Member] Contractual Interest Rate Reduction And Extended Maturity [Member] Payment Deferral Payment Deferral [Member] Other Other Loan Restructuring Modifications [Member] Other Loan Restructuring Modifications [Member] Commercial business Financing Receivable, Modifications [Line Items] Financing Receivable, Modifications [Line Items] Loan balances modified as TDRs Financing Receivable, Modifications, Post-Modification Recorded Investment Statement of Financial Position [Abstract] Allowance for loan losses, loans receivable Loans and Leases Receivable, Allowance 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 Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] 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] Employee Stock Option Employee Stock Option [Member] Stock Appreciation Rights (SARs) Stock Appreciation Rights (SARs) [Member] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Total unrecognized compensation cost related to outstanding stock options Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized Unrecognized cost related to outstanding stock options, period of recognition Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for 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) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Schedule of Assets Sold under Agreements to Repurchase [Table] Schedule of Assets Sold under Agreements to Repurchase [Table] Financial Instrument [Axis] Financial Instrument [Axis] Financial Instruments [Domain] Financial Instruments [Domain] U.S. Agency Securities US Treasury and Government [Member] 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 Earnings Per Share [Abstract] Net income available to common stockholders Net Income (Loss) Attributable to Parent 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) Weighted Average Number of Shares Outstanding, Basic Dilutive effect of stock options (in shares) Dilutive Effect Of Stock Options Dilutive effect of stock options. Weighted-average diluted shares (in shares) Weighted Average Number of Shares Outstanding, Diluted 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 Anti-dilutive stock options excluded from diluted earnings per share calculation (in shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Loans Receivable and Allowance for Loan Losses Loans, Notes, Trade and Other Receivables Disclosure [Text Block] Changes in the Carrying Amount of Goodwill and Core Deposit Intangible Assets Schedule of Intangible Assets and Goodwill [Table Text Block] Marketable equity securities Equity Securities [Member] Investment Type [Axis] Investment Type [Axis] Investments [Domain] Investments [Domain] Industrial Industrial [Member] Industrial. Oil and gas Oil and Gas Properties [Member] Available-for-sale, Debt Securities Available-for-sale Debt Securities, Amortized Cost Basis [Abstract] Gross Unrealized Gains Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax Gross Unrealized Losses Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax Available-for-sale, Equity Securities 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, Equity Securities Available-for-sale Securities, Total Available-for-sale Securities, Fair Value to 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 Available-for-sale securities - at fair value Held-to-maturity, Debt Securities Held-to-maturity Securities [Abstract] Amortized Cost Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment Gross Unrealized Gains Held-to-maturity Securities, Accumulated Unrecognized Holding Gain Gross Unrealized Losses Held-to-maturity Securities, Accumulated Unrecognized Holding Loss Fair Value Number of Contracts Financing Receivable, Modifications, Number of Contracts Pre-Modification Outstanding Recorded Investment Financing Receivable, Modifications, Pre-Modification Recorded Investment Post-Modification Outstanding Recorded Investment 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] Securities Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] Schedule of Impaired Financing Receivable [Table] Schedule of Impaired Financing Receivable [Table] Unallocated Unallocated Financing Receivables [Member] Receivables Acquired with Deteriorated Credit Quality [Axis] Receivables Acquired with Deteriorated Credit Quality [Axis] Receivables Acquired with Deteriorated Credit Quality [Domain] Receivables Acquired with Deteriorated Credit Quality [Domain] Loans acquired with deteriorated credit quality Receivables Acquired with Deteriorated Credit Quality [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 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 Fair Value Measurements, Recurring and Nonrecurring [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Nonrecurring Fair Value, Measurements, Nonrecurring [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] 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. Statement of Stockholders' Equity [Abstract] Statement [Table] Statement [Table] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Retained Earnings Retained Earnings [Member] Statement [Line Items] Statement [Line Items] Dividends paid per common share (in usd per share) Common Stock, Dividends, Per Share, Cash Paid 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 Provision for Loan and Lease 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 Goodwill and Core Deposit Intangibles Goodwill and Intangible Assets Disclosure [Text Block] Fair Value Measurements Fair Value Disclosures [Text Block] 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 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 Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Restricted Stock Restricted Stock [Member] Number of Shares Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] Unvested as of December 31, 2017 (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Vested (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period Forfeited (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period Unvested as of March 31, 2018 (in shares) Weighted-Average Grant-Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] Unvested as of December 31, 2017 (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 Granted (in usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value Vested (in usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value Forfeited (in usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value Unvested as of March 31, 2018 (in usd per share) Goodwill 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 Amortization expense Amortization of Intangible Assets Balance at End of Period Regulatory Matters Regulatory Capital Requirements under Banking Regulations [Text Block] Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Table] Fair Value Measurements, Recurring and Nonrecurring, 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] 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 Fair Value, Option, Qualitative Disclosures Related to Election [Line Items] Fair Value, Option, Qualitative Disclosures Related to Election [Line Items] 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. Federal home loan bank advances interest rate at period end Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Interest Rate at Period End 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 Statement of Comprehensive Income [Abstract] 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), Unrealized Holding Gain (Loss) on Securities Arising During Period, before Tax Reclassification adjustment for 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), Available-for-sale Securities 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), Available-for-sale Securities Adjustment, Net of Tax Interest rate swaps designated as cash flow hedges: Interest Rate Cash Flow Hedges [Abstract] Unrealized gains Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax Reclassification adjustment for losses recognized in interest expense Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax Net unrealized gains Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, before Tax Tax effect - expense Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax Net-of-tax amount - interest rate swaps Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of 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 Comprehensive Income (Loss), Net of Tax, Attributable to Parent Equity [Abstract] Accumulated Other Comprehensive Income (Loss) [Table] Accumulated Other Comprehensive Income (Loss) [Table] 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 Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] AOCI Attributable to Parent AOCI Attributable to Parent [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 Total stockholders’ equity Stockholders' Equity Attributable to Parent Reclassification from accumulated other comprehensive loss to retained earnings 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 Plan Name [Axis] Plan Name [Axis] Plan Name [Domain] Plan Name [Domain] 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 Employee Stock Ownership Plan (ESOP), Compensation Expense 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 Stock-based compensation expense Allocated Share-based Compensation Expense ESOP expense Provision for loan losses Gains on sales of securities, net Gain (Loss) on Sale of Securities, Net Net unrealized loss 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 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 (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] 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 Available-for-sale Securities Principal payments on available-for-sale securities Proceeds from Sale and Maturity of Available-for-sale Securities 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 Available-for-sale Securities 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 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 Proceeds from (Repayments of) Restricted Cash, 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 Federal Home Loan Bank Advances Proceeds from long-term FHLBB advances Proceeds from Federal Home Loan Bank Advances 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 decrease in cash and cash equivalents Cash and Cash Equivalents, Period Increase (Decrease) Cash and cash equivalents, beginning of period Cash and Cash Equivalents, at Carrying Value 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 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 Derivatives and Hedging Activities Derivative Instruments and Hedging Activities Disclosure [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 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 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), Available-for-sale Securities Adjustment, Net of Tax Increase to retained earnings Retained Earnings (Accumulated Deficit) Decrease to accumulated other comprehensive income Accumulated Other Comprehensive Income (Loss), Net of Tax Unrealized losses, marketable equity securities Equity Securities, Fv-Ni, Unrealized Loss Equity Securities, Fv-Ni, Unrealized 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 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 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 Obligations Of Political Subdivisions Obligations of political subdivisions. Available-for-sale securities with unrealized losses, less than twelve months Debt Securities, Available-For-Sale, Continuous Unrealized Loss Position, Less Than 12 Months, Number Of Positions Debt Securities, Available-For-Sale, Continuous Unrealized Loss Position, Less Than 12 Months, Number Of Positions Percentage of continuous unrealized losses to amortized cost basis for less than twelve months Debt Securities, Available-For-Sale Securities, Continuous Unrealized Loss Position, Less Than 12 Months, Unrealized Losses As Percentage Of Amortized Cost Basis Debt Securities, Available-For-Sale Securities, Continuous Unrealized Loss Position, Less Than 12 Months, Unrealized Losses As Percentage Of Amortized Cost Basis Securities with unrealized losses for twelve months or more Debt Securities, Available-For-Sale, Continuous Unrealized Loss Position, 12 Months Or Longer, Number Of Positions Debt Securities, Available-For-Sale, Continuous Unrealized Loss Position, 12 Months Or Longer, Number Of Positions Percentage of continuous unrealized losses to amortized cost basis for twelve months or more Debt Securities, Available-For-Sale, Continuous Unrealized Loss Position, 12 Months Or Longer, Unrealized Losses As Percentage Of Amortized Cost Basis Debt Securities, Available-For-Sale, Continuous Unrealized Loss Position, 12 Months Or Longer, Unrealized Losses As Percentage Of Amortized Cost Basis Number of held to maturity securities with unrealized losses Held-to-maturity, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions Unrealized losses on debt securities held-to-maturity Other-than-temporary impairment losses Other than Temporary Impairment Losses, Investments 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. Schedule of Basic and Diluted Net Income Per Share Schedule of Weighted Average Number of Shares [Table Text Block] Schedule of Compliance with Regulatory Capital Requirements under Banking Regulations [Table] Schedule of Compliance with Regulatory Capital Requirements under Banking Regulations [Table] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] United Bank United Bank [Member] United Bank [Member] United Financial Bancorp, Inc. United Financial Bancorp Inc [Member] United Financial Bancorp Inc. [Member] Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] Total capital to risk weighted assets Capital [Abstract] Total capital to risk weighted assets, Actual, Amount Capital Total capital to risk weighted assets, Actual, Ratio Capital to Risk Weighted Assets Total capital to risk weighted assets, Minimum For Capital Adequacy Purposes, Amount Capital Required for Capital Adequacy Total capital to risk weighted assets, Minimum For Capital Adequacy Purposes, Ratio Capital Required for Capital Adequacy to Risk Weighted Assets Total capital to risk weighted assets, Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount Capital Required to be Well Capitalized 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 Common equity tier 1 capital to risk weighted assets Tier One Common Equity Risk Based Capital [Abstract] Tier One Common Equity Risk Based Capital [Abstract] Common equity tier 1 capital to risk weighted assets, Actual, Amount Common Equity Tier 1 Risk Based Capital Common Equity Tier 1 Risk Based Capital Common equity tier 1 capital to risk weighted assets, Actual, Ratio Common Equity Tier 1 Risk Based Capital To Risk Weighted Assets Common Equity Tier 1 Risk Based Capital To Risk Weighted Assets Common equity tier 1 capital to risk weighted assets, Minimum For Capital Adequacy Purposes, Amount Common Equity Tier 1 Risk Based Capital Required For Capital Adequacy Common Equity Tier 1 Risk Based Capital Required For Capital Adequacy Common equity tier 1 capital to risk weighted assets, Minimum For Capital Adequacy Purposes, Ratio Common Equity Tier 1 Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Common Equity Tier 1 Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Common equity tier 1 capital to risk weighted assets, Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount Common Equity Tier 1 Risk Based Capital Required To Be Well Capitalized Common Equity Tier 1 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, Ratio Common Equity Tier 1 Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets Common Equity Tier 1 Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets Tier 1 capital to risk weighted assets Tier One Risk Based Capital [Abstract] Tier 1 capital to risk weighted assets, Actual, Amount Tier One Risk Based Capital Tier 1 capital to risk weighted assets, Actual, Ratio Tier One Risk Based Capital to Risk Weighted Assets Tier 1 capital to risk weighted assets, Minimum For Capital Adequacy Purposes, Amount Tier One Risk Based Capital Required for Capital Adequacy Tier 1 capital to risk weighted assets, Minimum For Capital Adequacy Purposes, Ratio Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets 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 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 Tier 1 capital to total average assets Tier One Leverage Capital [Abstract] Tier 1 capital to total average assets, Actual, Amount Tier One Leverage Capital Tier 1 capital to total average assets, Actual, Ratio Tier One Leverage Capital to Average Assets Tier 1 capital to total average assets, Minimum For Capital Adequacy Purposes, Amount Tier One Leverage Capital Required for Capital Adequacy Tier 1 capital to total average assets, Minimum For Capital Adequacy Purposes, Ratio Tier One Leverage Capital Required for Capital Adequacy to Average Assets Tier 1 capital to total average assets, Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount Tier One Leverage Capital Required to be Well Capitalized Tier 1 capital to total 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 Amount available for the payment of dividends Statutory Accounting Practices, Statutory Amount Available for Dividend Payments with Regulatory Approval ASSETS Assets [Abstract] Cash and due from banks Cash and Due from Banks Short-term investments Short-term Investments Total cash and cash equivalents Available-for-sale securities - at fair value Held-to-maturity securities - at amortized cost Held-to-maturity Securities Loans held for sale Loans Receivable Held-for-sale, Net, Not Part of Disposal Group Loans receivable (net of allowance for loan losses of $47,915 at March 31, 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 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; 50,976,667 and 51,044,752 shares issued and outstanding, at March 31, 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 Accumulated other comprehensive loss, net of tax Total liabilities and stockholders’ equity Liabilities and Equity 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 Expected future capital contribution commitments Contractual Obligation, Expected Contractual Obligation, Expected 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 Derivative Instruments, Gain (Loss) [Table] Derivative Instruments, Gain (Loss) [Table] Mortgage servicing rights derivative Income Statement Location [Axis] Income Statement Location [Axis] Income Statement Location [Domain] Income Statement Location [Domain] Interest income Interest Income [Member] Derivative Instruments, Gain (Loss) [Line Items] Derivative Instruments, Gain (Loss) [Line Items] Amount of Gain Recognized in AOCI (Effective Portion) Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net Amount of Losses Reclassified from AOCI into Expense (Effective Portion) Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net Amount of Gain (Loss) Recognized in Income from Derivatives Derivative, Gain (Loss) on Derivative, Net Amount of Gain Recognized in Income from Hedged Items Derivative Instruments Gain Loss Recognized In Income On Related Hedged Items Net Derivative Instruments Gain Loss Recognized In Income On Related Hedged Items Net Available for Sale Securities Schedule of Available-for-sale Securities Reconciliation [Table Text Block] Held-to-maturity Securities Held-to-maturity Securities [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] 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] 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 2015 Omnibus Stock Incentive Plan Two Thousand Fifteen Omnibus Stock Incentive Plan [Member] Two Thousand Fifteen Omnibus Stock Incentive Plan [Member] Restricted stock issued (in shares) Weighted average grant date fair value (in usd per share) Unvested restricted stock, period of recognition Total unrecognized compensation cost related to unvested restricted stock Financial Instruments Contract Amounts Represent Credit Risk Schedule of Fair Value, Off-balance Sheet Risks [Table Text Block] 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 Tax benefit recorded Employee Service Share-based Compensation, Tax Benefit from Compensation Expense Other non-interest expenses Other Noninterest Expense Mortgage Loans on Real Estate, Loan Type [Axis] Mortgage Loans on Real Estate, Loan Type [Axis] Mortgage Loans on Real Estate, Loan Type [Domain] Mortgage Loans on Real Estate, Loan Type [Domain] Mortgage loan derivative assets and liabilities Conventional Mortgage Loan [Member] Available-for-Sale Securities: Total available-for-sale debt securities Marketable 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 Interest rate swap assets Interest Rate Derivative Assets, at Fair Value Interest rate swap liabilities Interest Rate Derivative Liabilities, at Fair Value 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] Estimated reduction/increase to interest expense Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net