Investments, Debt and Equity Securities [Abstract] Net unrealized gain (loss) on securities available for sale Available-for-sale Securities, 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 Encumbered securities Encumbered Securities Encumbered securities. Fair value pledged for derivative collateral Trading Securities Pledged as Collateral 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 Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year Unrealized losses equaling cost basis for less than twelve months (percent) Available For Sale Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Aggregate Losses As Percentage Of Cost Basis Available for sale securities continuous unrealized loss position less than twelve months aggregate losses as percentage of cost basis. Securities with unrealized losses for twelve months or more Securities In Unrealized Loss Positions, Qualitative Disclosure, Number Of Positions Greater Than Or Equal To One Year Securities In Unrealized Loss Positions, Qualitative Disclosure, Number Of Positions Greater Than Or Equal To One Year Unrealized losses equaling cost basis for twelve months or more (percent) Securities In Unrealized Loss Position, Twelve Months Or More Aggregate Losses As Percentage Of Cost Basis Securities In Unrealized Loss Position, Twelve Months Or More Aggregate Losses As Percentage Of Cost Basis Number of issues had unrealized losses for less than twelve months Securities In Unrealized Loss Positions, Qualitative Disclosure, Number Of Positions, Less Than One Year Securities In Unrealized Loss Positions, Qualitative Disclosure, Number Of Positions, Less Than One Year Number of issues had unrealized losses for twelve months or more Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year Individual unrealized loss that represent an other than temporary impairment Other than Temporary Impairment Losses, Investments Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract] Components of Other Assets Schedule of Other Assets [Table Text Block] Commitments and Contingencies Disclosure [Abstract] Contractual Obligation By Type [Table] Contractual Obligation By Type [Table] Contractual Obligation By Type. Financial Instrument [Axis] Financial Instrument [Axis] Financial Instruments [Domain] Financial Instruments [Domain] Commitments to extend credit Commitments to Extend Credit [Member] Concentration of Credit or Market Risk [Axis] Concentration of Credit or Market Risk [Axis] Fair Value, Concentration of Risk, Disclosure Items [Domain] Fair Value, Concentration of Risk, Disclosure Items [Domain] Commitment to grant loans Commitment To Grant Loans [Member] Commitment To Grant Loans. Undisbursed construction loans Undisbursed Construction Loans [Member] Undisbursed Construction Loans. Undisbursed home equity lines of credit Undisbursed Home Equity Lines Of Credit [Member] Undisbursed Home Equity Lines Of Credit. Undisbursed commercial lines of credit Undisbursed Commercial Lines Of Credit [Member] Undisbursed Commercial Lines Of Credit. Standby letters of credit Standby Letters of Credit [Member] Unused credit card lines Unused Credit Card Lines [Member] Unused Credit Card Lines [Member] Unused checking overdraft lines of credit Unused Checking Overdraft Lines Of Credit [Member] Unused Checking Overdraft Lines Of Credit. Contractual Obligation By Type [Line Items] Contractual Obligation By Type [Line Items] Contractual Obligation By Type Commitments to extend credit: Loss Contingency, Estimate [Abstract] Fair value disclosure, off-balance sheet risks, amount, liability Fair Value Disclosure, Off-balance Sheet Risks, Amount, Liability 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 OTHER ASSETS Other Assets Disclosure [Text Block] Retirement Benefits [Abstract] Schedule of Defined Benefit Plans Disclosures [Table] Schedule of Defined Benefit Plans Disclosures [Table] Retirement Plan Type [Axis] Retirement Plan Type [Axis] Retirement Plan Type [Domain] Retirement Plan Type [Domain] Qualified Pension Plan Pension Plan [Member] Supplemental Executive Retirement Plans Supplemental Employee Retirement Plan [Member] Other Post - Retirement Benefits Other Postretirement Benefits Plan [Member] Defined Benefit Plan Disclosure [Line Items] Defined Benefit Plan Disclosure [Line Items] Discount rate Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate Expected return on plan assets Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets Rate of compensation increase Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase Equity [Abstract] ACCUMULATED OTHER COMPREHENSIVE LOSS Comprehensive Income (Loss) Note [Text Block] Quarterly Financial Information Disclosure [Abstract] Quarterly Financial Information of Company Quarterly Financial Information [Table Text Block] Business Combinations [Abstract] Schedule of Business Acquisitions, by Acquisition [Table] Schedule of Business Acquisitions, by Acquisition [Table] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Legacy United Legacy United [Member] Legacy United [Member] Entity by Location [Axis] Entity by Location [Axis] Location [Domain] Location [Domain] Express Drive-Up Drive Up Office [Member] Drive Up Office [Member] Loan Production Offices Loan Production Office [Member] Loan Production Office [Member] Loans At Acquisition Date [Line Items] Business Acquisition [Line Items] Acquired common shares outstanding percentage Business Acquisition, Percentage of Voting Interests Acquired Number of branch location Number Of Branch Locations Number Of Branch Locations Shares issued to acquiree entity (in shares) Conversion of Stock, Shares Issued Total consideration paid at closing Business Combination, Consideration Transferred Closing price of the common share (usd per share) Business Acquisition, Share Price 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] 2017 Federal Home Loan Bank, Advances, Maturities Summary, Due in Remainder of Fiscal Year 2018 Federal Home Loan Bank, Advances, Maturities Summary, Due in Next Twelve Months 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 Total federal home loan bank advances Federal Home Loan Bank Advances And Other Borrowings Federal Home Loan Bank Advances And Other Borrowings Weighted-Average rate in 2017 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 2018 Federal Home Loan Bank Advances Branch Of Fhlb Bank Weighted Average Interest Rate Within One Year Federal home loan bank advances branch of FHLB bank weighted average interest rate due within one 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 Income Tax Disclosure [Abstract] Operating Loss Carryforwards [Table] Operating Loss Carryforwards [Table] Geographical [Axis] Geographical [Axis] Geographical [Domain] Geographical [Domain] Connecticut CONNECTICUT Tax Credit Carryforward [Axis] Tax Credit Carryforward [Axis] Tax Credit Carryforward, Name [Domain] Tax Credit Carryforward, Name [Domain] Investment Tax Credit Carryforward Investment Tax Credit Carryforward [Member] Operating Loss Carryforwards [Line Items] Operating Loss Carryforwards [Line Items] Federal statutory income tax rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent Federal net operating loss Deferred Tax Assets, Operating Loss Carryforwards, Domestic Valuation allowance Deferred Tax Assets, Valuation Allowance State net operating loss Deferred Tax Assets, Operating Loss Carryforwards, State and Local Tax credit carryforward Tax Credit Carryforward, Amount Contingency reserve for loan losses Contingency Reserve For Loan Losses Contingency reserve for Loan and Lease Losses. Unrecognized deferred income taxes Unrecognized Deferred Income Tax Unrecognized deferred income tax. Uncertain tax positions Liability for Uncertainty in Income Taxes, Current Accrued interest and penalties Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued Provisional tax expense, Tax Act Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Provisional Income Tax Expense (Benefit) Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Provisional Income Tax Expense (Benefit) Fair Value Disclosures [Abstract] Fair Value Measurements, Recurring and Nonrecurring [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Fair Value, Hierarchy [Axis] Fair Value, Hierarchy [Axis] Fair Value Hierarchy [Domain] Fair Value Hierarchy [Domain] Significant Unobservable Inputs (Level 3) Fair Value, Inputs, Level 3 [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] Residential real estate mortgage loans held for sale Loans Receivable Held-for-sale, Net, Not Part of Disposal Group, Mortgage Loans held for sale, fair value Loans Held-for-sale, Fair Value Disclosure 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 Number of pooled trust securities Number Of Pooled Trust Securities Number Of Pooled Trust Securities Fixed rate mortgage loans term (in years) Fixed Rate Mortgage Loans Term Fixed rate mortgage loans term. Maturity period, term 1 (in years) Fixed Rate Mortgage Loans Term One Fixed rate mortgage loans term one. Maturity period, term 2 (in years) Fixed Rate Mortgage Loans Term Two Fixed rate mortgage loans term two. Maturity period, term 3 (in years) Fixed Rate Mortgage Loans Term Three Fixed rate mortgage loans term three. Estimates fallout rate based upon historical average (percent) 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 Short-term investments, maturity period Short Term Investments Maturity Period Short-term investments, maturity period. Property, Plant and Equipment [Abstract] Premises and Equipment Property, Plant and Equipment [Table Text Block] Disclosure of Compensation Related Costs, Share-based Payments [Abstract] SHARE-BASED COMPENSATION PLANS Disclosure of Compensation Related Costs, Share-based Payments [Text Block] Demand and NOW Demand Deposit Accounts Regular savings Deposits, Savings Deposits Money markets Deposits Money Market And Investment Savings Deposits Deposits money market and investment savings deposits. Time deposits Time Deposits Total deposits Deposits Goodwill and Intangible Assets Disclosure [Abstract] 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 deposit intangibles Core Deposits [Member] Finite-Lived Intangible Assets [Line Items] Finite-Lived Intangible Assets [Line Items] Core deposits, estimated useful life Finite-Lived Intangible Asset, Useful Life Core deposit intangible amortization Amortization of Intangible Assets 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] Stock Option Employee Stock Option [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] Title of Individual [Axis] Title of Individual [Axis] Relationship to Entity [Domain] Relationship to Entity [Domain] Employee and Director Employee and Director [Member] Employee and Director [Member] Director Director [Member] Officer Officer [Member] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Shares authorized under the plan (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized Stock awards, granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross Restricted shares granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period 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 Share-based compensation expense Allocated Share-based Compensation Expense Tax benefit recorded Employee Service Share-based Compensation, Tax Benefit from Compensation Expense Share-based compensation Share-based Compensation Number of Stock Options (shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] Outstanding at December 31, 2016 (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number Granted (in shares) Exercised (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Forfeited, expired or canceled (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures in Period Outstanding at December 31, 2017 (in shares) Stock options vested and exercisable at December 31, 2017 (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 (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract] Outstanding at December 31, 2016 (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price Granted (usd per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price Exercised (usd per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price Forfeited or expired (usd per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Forfeitures in Period, Weighted Average Exercise Price Outstanding at December 31, 2017 (usd per share) Stock options vested and exercisable at December 31, 2017 (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 December 31, 2017 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 December 31, 2017 Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term Aggregate Intrinsic Value, Exercised Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value Aggregate Intrinsic Value, Outstanding at December 31, 2017 Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value Aggregate Intrinsic Value, Stock options vested and exercisable at December 31, 2017 Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value FAIR VALUE MEASUREMENT Fair Value Disclosures [Text Block] Summary of Deposits Schedule Of Components Of Deposits By Type Table [Table Text Block] Schedule of components of deposits by type. Summary of Contractual Maturities of Time Deposits Schedule Of Contractual Maturities Of Time Deposits Table [Table Text Block] Schedule of contractual maturities of time deposits. 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. 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] 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] Investment Type [Axis] Investment Type [Axis] Investments [Domain] Investments [Domain] Banks Banks [Member] Banks. 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: Available-for-sale Securities [Abstract] Debt securities: Available-for-sale Debt Securities, Amortized Cost Basis [Abstract] Amortized Cost Available-for-sale Debt Securities, Amortized Cost Basis 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 Fair Value Available-for-sale Securities, Debt Securities Marketable equity securities, by sector: 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 Total available for sale securities 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 Fair Value Available-for-sale Securities Held to maturity: 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 Held to maturity securities Held-to-maturity Securities, Fair Value Earnings Per Share [Abstract] NET INCOME PER SHARE Earnings Per Share [Text Block] Statement of Comprehensive Income [Abstract] Income tax expense associated with the reclassification adjustment for gains realized in net income Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax Income tax benefit associated with the reclassification adjustment from AOCI for borrowed funds Other Comprehensive Income (Loss), Reclassification Adjustment From AOCI For Borrowed Funds, Tax Other Comprehensive Income (Loss), Reclassification Adjustment From AOCI For Borrowed Funds, Tax Income tax benefit associated with the reclassification adjustment for prior service costs under post-retirement plans Other Comprehensive Income Loss Reclassification Postretirement Benefit Plans Net Transition Asset Obligation Recognized In Net Periodic Benefit Cost Tax Other Comprehensive Income Loss Reclassification Postretirement Benefit Plans Net Transition Asset Obligation Recognized In Net Periodic Benefit Cost Tax Income tax benefit associated with the reclassification adjustment for losses (benefit) recognized in net periodic benefit cost under post-retirement plans 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 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 expense 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 Bank Servicing Fees 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 Gain on sales of securities, net Gain (Loss) on Sale of Securities, Net 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 Occupancy and equipment Occupancy, Net Service bureau fees Service Bureau Fees Service bureau fees. Professional fees Professional Fees Marketing and promotions Marketing and Advertising Expense FDIC insurance assessments Federal Deposit Insurance Corporation Premium Expense Merger related expense Payment For Merger Related Costs Payment for Merger Related Costs FHLBB prepayment penalties Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Repayment and Penalties 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 (usd per share) Earnings Per Share, Basic Diluted (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 Goodwill Goodwill [Roll Forward] Beginning balance Goodwill Adjustments Goodwill, Period Increase (Decrease) Ending balance Core Deposit Intangible Finite-lived Intangible Assets [Roll Forward] Beginning balance Finite-Lived Core Deposits, Gross Adjustments Finite-Lived Intangible Assets, Period Increase (Decrease) Amortization expense Ending balance Accounting Policies [Abstract] 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, Receivables-Nonrefundable Fees and Other Costs 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 Net income Adjusted weighted-average common shares outstanding (in shares) Weighted Average Number of Shares Issued, Basic Less: average number of unvested 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) Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Change in Benefit Obligation: Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] Benefit obligation at beginning of year Defined Benefit Plan, Benefit Obligation Service cost Defined Benefit Plan, Service Cost Interest cost Defined Benefit Plan, Interest Cost Plan participants’ contributions Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant Actuarial loss (gain) Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) Benefits paid and administration expenses Defined Benefit Plan, Benefit Obligation, Benefits Paid Curtailments, settlements, special termination benefits Defined Benefit Plan Curtailments Settlements Special Termination Benefits Changes Benefit Obligation Defined Benefit Plan Curtailments Settlements Special Termination Benefits Changes Benefit Obligation Benefit obligation at end of year Change in Plan Assets: Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] Fair value of plan assets at beginning of year Defined Benefit Plan, Fair Value of Plan Assets Actual return (loss) on plan assets Defined Benefit Plan, Plan Assets, Increase (Decrease) for Actual Return (Loss) Employer contributions Defined Benefit Plan, Plan Assets, Contributions by Employer Plan participants’ contributions Defined Benefit Plan, Plan Assets, Contributions by Plan Participant Benefits paid and administration expenses Defined Benefit Plan, Plan Assets, Benefits Paid Settlements Defined Benefit Plan, Plan Assets, Payment for Settlement Fair value of plan assets at end of year Funded Status: Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] Underfunded status at end of year Defined Benefit Plan, Funded (Unfunded) Status of Plan Amounts Recognized in the Consolidated Statements of Condition Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] Accrued expenses and other liabilities Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position Derivative Instruments in Hedges, at Fair Value, Net [Abstract] Derivative Instruments, Gain (Loss) [Table] Derivative Instruments, Gain (Loss) [Table] Hedging Designation [Axis] Hedging Designation [Axis] Hedging Designation [Domain] Hedging Designation [Domain] Designated as Hedging Instrument Designated as Hedging Instrument [Member] Not Designated as Hedging Instrument Not Designated as Hedging Instrument [Member] Derivative Instrument [Axis] Derivative Instrument [Axis] Derivative Contract [Domain] Derivative Contract [Domain] Derivative loan commitments Derivative Loan Commitments [Member] Derivative Loan Commitments [Member] Mortgage servicing rights derivative Mortgage Loan Servicing Rights [Member] Mortgage Loan Servicing Rights [Member] Forward loan sale commitments Forward Loan Sale Commitments [Member] Forward Loan Sale Commitments [Member] Interest rate swaps Interest Rate Swap [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] 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 Loss Recognized in OCI on Derivatives (Effective Portion) Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net Amount of Loss Reclassified from AOCI into Income (Effective Portion) Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net Amount of Gain (Loss) Recognized in Income on Derivatives Derivative, Gain (Loss) on Derivative, Net Amount of Gain (Loss) Recognized in Income on 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 Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Table Text Block] Schedule of Assets Recorded at Fair Value on Recurring Basis Fair Value, Assets 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] Fair Value Inputs, Assets, Quantitative Information 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 Losses on Assets Recorded at Fair Value on Non-Recurring Basis Gains Losses Included In Earnings Fair Value Disclosure Non Recurring Basis 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] Receivables [Abstract] Changes in Loans Outstanding, Related Parties Loans and Leases Receivable, Related Parties [Roll Forward] Balance, beginning of year Loans and Leases Receivable, Related Parties Loans related to parties who terminated service during the year Loans And Leases Receivable Related Parties Loan To Parties Terminated From Service Loans and leases receivable related parties loan to parties terminated from service. Additional loans and advances Loans and Leases Receivable, Related Parties, Additions Repayments Loans and Leases Receivable, Related Parties, Proceeds Balance, end of year Estimated amortization expense for the years ending December 31, Finite-Lived Intangible Assets, Amortization Expense, Maturity Schedule [Abstract] 2018 Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months 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 Core deposit intangible 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] Minimum Minimum [Member] Maximum Maximum [Member] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] 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] Subordinated Notes Due October 2024 Subordinated Notes Due October 2024 [Member] Subordinated Notes Due October 2024 [Member] Brokered Sweep Deposit Brokered Deposits [Member] Brokered Deposits [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] 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 Unused 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. Percent of aggregate principal amount Threshold Percentage Of Investment In Capital Stock Based On Aggregate Unpaid Principal Amount Threshold Percentage Of Investment In Capital Stock Based On Aggregate Unpaid Principal Amount. Borrowings from the FHLBB (percent) Investment In Capital Stock As Percentage Of Federal Home Loan Bank Advances Investment In Capital Stock As Percentage of Federal Home Loan Bank Advances. Federal Home Loan Bank Stock Federal Home Loan Bank Stock 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 Wholesale reverse repurchase agreements, remaining term Debt Instrument Remaining Term Debt Instrument Remaining Term Weighted average cost (percent) Debt, Weighted Average Interest Rate Investment securities pledged as collateral Securities Held as Collateral, at Fair Value Subordinated notes, face amount Debt Instrument, Face Amount Subordinated notes, stated interest rate Debt Instrument, Interest Rate, Stated Percentage 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 variable rate Debt Instrument, Basis Spread on Variable Rate Percentage of other borrowings at cost Debt Instrument, Interest Rate, Effective Percentage Number of leased bank branches Number Of Leased Bank Branches Number Of Leased Bank Branches Capital lease obligations Capital Lease Obligations Other borrowings Brokered Deposit Brokered Deposit Number of counterparties to unused federal funds lines of credit Number Of Counterparties, Lines Of Credit Number Of Counterparties, Lines Of Credit Unused federal funds lines of credit Line of Credit Facility, Maximum Borrowing Capacity Number of Shares, Restricted Stock Activity (shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] Beginning balance (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Granted (in shares) 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 Ending balance (in shares) Restricted Stock Options, Weighted Average Grant Date Fair Value (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] Beginning balance (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 (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 (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 (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value Ending balance (usd per share) Operating Leases, Future Minimum Payments Receivable [Abstract] 2018 Operating Leases, Future Minimum Payments Receivable, Current 2019 Operating Leases, Future Minimum Payments Receivable, in Two Years 2020 Operating Leases, Future Minimum Payments Receivable, in Three Years 2021 Operating Leases, Future Minimum Payments Receivable, in Four Years 2022 Operating Leases, Future Minimum Payments Receivable, in Five Years Future minimum rental receivable under the non-cancelable leases Operating Leases, Future Minimum Payments Receivable Amount available for the payment of dividends Statutory Accounting Practices, Statutory Amount Available for Dividend Payments with Regulatory Approval Components of Net Periodic Benefit Cost: Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] Expected return on plan assets Defined Benefit Plan, Expected Return (Loss) on Plan Assets Amortization of net actuarial losses Defined Benefit Plan, Amortization of Gain (Loss) Amortization of prior service cost Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Settlement charge Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement Net periodic benefit cost Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income: Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract] Net loss (gain) Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, after Tax Amortization of net loss Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, after Tax Amortization of prior service cost Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, after Tax Loss recognized due to settlement Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), after Tax Total recognized in other comprehensive income (loss) Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax Total recognized in net periodic benefit cost and other comprehensive income (loss) Defined Benefit Plan Net Periodic Benefit Cost And Other Comprehensive Income Loss Recognized Amount Defined benefit plan net periodic benefit cost and other comprehensive income loss recognized amount. Schedule of Financing Receivable, Troubled Debt Restructurings [Table] Schedule of Financing Receivable, Troubled Debt Restructurings [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] Financing Receivable, Modifications [Line Items] Financing Receivable, Modifications [Line Items] Impaired Financing Receivable, Recorded Investment [Abstract] Impaired Financing Receivable, Recorded Investment [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 REGULATORY MATTERS Regulatory Capital Requirements under Banking Regulations [Text Block] Mortgage servicing rights: Servicing Asset at Amortized Cost, Balance [Roll Forward] Balance at beginning of year Servicing Asset at Amortized Cost Change in fair value recognized in income Servicing Asset At Amortized Value Gain (Loss) Recognized In Net Income Servicing Asset at Amortized Value Gain (Loss) Recognized in Net Income Issuances/additions Servicing Asset at Amortized Cost, Additions Balance at end of year OTHER COMMITMENTS AND CONTINGENCIES Commitments and Contingencies Disclosure [Text Block] 2018 Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months 2019 Defined Benefit Plan, Expected Future Benefit Payment, Year Two 2020 Defined Benefit Plan, Expected Future Benefit Payment, Year Three 2021 Defined Benefit Plan, Expected Future Benefit Payment, Year Four 2022 Defined Benefit Plan, Expected Future Benefit Payment, Year Five Years 2023-2027 Defined Benefit Plan, Expected Future Benefit Payment, Five Fiscal Years Thereafter 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 Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] Accumulated Other Comprehensive Loss AOCI Attributable to Parent [Member] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Unrealized gain (loss) AOCI before Tax, Attributable to Parent Tax effect AOCI Tax, Attributable to Parent Total stockholders’ equity Stockholders' Equity Attributable to Parent Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Nonrecurring Fair Value, Measurements, Nonrecurring [Member] 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 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] Provision for loan losses 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 Share-based compensation expense Share Based Compensation Expense Share-based compensation expense. ESOP expense Employee Stock Ownership Plan (ESOP), Compensation Expense Loss on extinguishment of debt Gain (Loss) on Extinguishment of Debt Tax effects of share-based awards Excess Tax Benefit from Share-based Compensation, Operating Activities Gains on sales of securities, net Loans originated for sale Payments for Origination and Purchases of Loans Held-for-sale Principal of balance of loan 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 adjustment 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 Gain on lease terminations Gain (Loss) on Contract Termination 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 benefit 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 Proceeds from Sale of 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. Purchase of bank-owned life insurance Payment to Acquire Life Insurance Policy, Investing Activities Proceeds from bank-owned life insurance death benefit Proceeds from Life Insurance Policy Surrender of bank-owned life insurance Proceeds From Surrender Of Bank-Owned Life Insurance Proceeds From Surrender Of Bank-Owned Life Insurance Receivable of bank-owned life insurance Increase In Receivable Of Bank Owned Life Insurance Increase In Receivable Of Bank Owned Life Insurance Proceeds from sales of equipment Proceeds from Sale of Other Productive Assets Purchases of premises and equipment Payments to Acquire Property, Plant, 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 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 change in short-term FHLBB advances Proceeds from (Repayments of) Short-term Debt Proceeds from long-term FHLBB advances Proceeds from Federal Home Loan Bank Borrowings Repayments of long-term FHLBB borrowings and penalty Payments For Long Term Federal Home Loan Bank Advances Repayment of long-term FHLBB advances. Prepayments of FHLBB Advances Repayments of Federal Home Loan Bank Borrowings Repayments of called FHLBB advances Payments For Called Federal Home Loan Bank Advances Payments For Called Federal Home Loan Bank Advances Net decrease in other borrowings, excluding proceeds from subordinated debt issuance Proceeds from (Repayments of) Other Debt 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 Tax effects of share-based awards Excess Tax Benefit from Share-based Compensation, Financing Activities Cash dividends paid on common stock Payments of Dividends Net cash provided by financing activities Net Cash Provided by (Used in) Financing Activities Net (decrease) increase in cash and cash equivalents Cash and Cash Equivalents, Period Increase (Decrease) Cash and cash equivalents - beginning of year Cash and Cash Equivalents, at Carrying Value Cash and cash equivalents - end of year 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 from Real Estate Owned Increase (decrease) in due to broker, investment purchases Due To Broker Investment Purchase Increase Decrease Due To Broker Investment Purchase Increase Decrease Decrease in due to broker, common stock buyback Increase (Decrease) in Payables under Repurchase Agreements Acquisition of non-cash assets and liabilities: Noncash or Part Noncash Acquisition, Value of Assets Acquired [Abstract] Cash and liquid assets Assets Designated To Deposits In Non Interest Earning Reserves Restricted Cash And Cash Equivalents Assets designated to deposits in non interest earning reserves restricted cash and cash equivalents. 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 loans 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 non-owner occupied 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] Owner-occupied CRE Mortgage Loans on Real Estate [Abstract] 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 GOODWILL AND CORE DEPOSIT INTANGIBLES Goodwill and Intangible Assets Disclosure [Text Block] Components of Income Tax Expense Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] Summary of Provision for Income Taxes Schedule of Effective Income Tax Rate Reconciliation [Table Text Block] Summary of Deferred Tax Assets and Deferred Tax Liabilities Schedule of Deferred Tax Assets and Liabilities [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] Consolidated Entities [Axis] Consolidated Entities [Axis] Consolidated Entities [Domain] Consolidated Entities [Domain] United Bank Subsidiaries [Member] United Financial Bancorp, Inc Parent Company [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] Actual, Amount Capital Actual, Ratio Capital to Risk Weighted Assets Minimum For Capital Adequacy Purposes, Amount Capital Required for Capital Adequacy Minimum For Capital Adequacy Purposes, Ratio Capital Required for Capital Adequacy to Risk Weighted Assets Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount Capital Required to be Well Capitalized 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] Actual, Amount Common Equity Tier 1 Risk Based Capital Common Equity Tier 1 Risk Based Capital Actual, Ratio Common Equity Tier 1 Risk Based Capital To Risk Weighted Assets Common Equity Tier 1 Risk Based 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 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 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 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] Actual, Amount Tier One Risk Based Capital Actual, Ratio Tier One Risk Based Capital to Risk Weighted Assets Minimum For Capital Adequacy Purposes, Amount Tier One Risk Based Capital Required for Capital Adequacy Minimum For Capital Adequacy Purposes, Ratio Tier One Risk Based Capital Required for Capital Adequacy 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 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] Actual, Amount Tier One Leverage Capital Actual, Ratio Tier One Leverage Capital to Average Assets Minimum For Capital Adequacy Purposes, Amount Tier One Leverage Capital Required for Capital Adequacy Minimum For Capital Adequacy Purposes, Ratio Tier One Leverage Capital Required for Capital Adequacy to Average Assets Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount Tier One Leverage Capital Required to be Well Capitalized Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Ratio Tier One Leverage Capital Required to be Well Capitalized to Average Assets Schedule of Assets Sold under Agreements to Repurchase [Table] Schedule of Assets Sold under Agreements to Repurchase [Table] US Treasury and Government 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] Overnight Maturity Overnight [Member] Up to 1 Year Maturity Up To 1 Year [Member] Maturity Up To 1 Year [Member] 1 - 3 Years Maturity 1 to 3 Years [Member] Maturity 1 to 3 Years [Member] 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 Number of Contracts Financing Receivable, Modifications, Subsequent Default, Number of Contracts Recorded Investment Financing Receivable, Modifications, Subsequent Default, Recorded Investment MERGER Business Combination Disclosure [Text Block] Total debt securities Debt Securities [Member] Available for sale: Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] Fair Value Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract] Less than 12 months Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value 12 Months or More Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value Total Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value Unrealized Loss Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract] Less than 12 months Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 12 Months or More Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss Total Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss Held to maturity: Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract] 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 12 Months or More 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 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 Summary of Merger Schedule of Business Acquisitions, by Acquisition [Table Text Block] Restricted stock issued (in shares) Aggregate fair value of vested options Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value Granted, weighted average grant date fair value (usd per share) Compensation cost not yet recognized Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized Cost not yet recognized, period for recognition Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition Unvested restricted stock expected year two (in shares) Share-Based Compensation Arrangement By Share-Based Payment Award, Equity Instruments Other Than Options, Number Of Shares Expected To Vest With In Two Years Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Expected To Vest With in Two Year Number. Unvested restricted stock expected year three (in shares) Share-Based Compensation Arrangement By Share-Based Payment Award, Equity Instruments Other Than Options, Number Of Shares Expected To Vest With In Three Years Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other Than Options, Number of Shares Expected to Vest With in Three Years Unvested restricted stock expected year four (in shares) Share-Based Compensation Arrangement By Share-Based Payment Award, Equity Instruments Other Than Options, Number Of Shares Expected To Vest With In Four Years Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other Than Options, Number of Shares Expected to Vest With in Four Years Unvested restricted stock expected year five (in shares) Share-Based Compensation Arrangement By Share-Based Payment Award, Equity Instruments Other Than Options, Number Of Shares Expected To Vest With In Five Years Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other Than Options, Number of Shares Expected to Vest With in Five Years LOANS RECEIVABLE AND ALLOANCE FOR LOAN LOSSES Loans, Notes, Trade and Other Receivables Disclosure [Text Block] Schedule of Impaired Financing Receivable [Table] Schedule of Impaired Financing Receivable [Table] Financing Receivable, Impaired [Line Items] Financing Receivable, Impaired [Line Items] Average Recorded Investment without a valuation allowance Impaired Financing Receivable, Average Recorded Investment Interest Income Recognized without a valuation allowance Impaired Financing Receivable, Interest Income, Accrual Method Nature of Operations and Financial Statement Presentation Basis of Accounting, Policy [Policy Text Block] Common Share Repurchases Stockholders' Equity, Policy [Policy Text Block] Cash and Cash Equivalents Cash and Cash Equivalents, Policy [Policy Text Block] Securities Marketable Securities, Policy [Policy Text Block] Derivative Financial Instruments Derivatives, Methods of Accounting, Hedge Effectiveness [Policy Text Block] Derivative Loan Commitments Loan Commitments, Policy [Policy Text Block] Forward Loan Sale Commitments Forward Loan Sale Commitments Policy [Policy Text Block] Forward loan sale commitments. Federal Home Loan Bank Stock Federal Home Loan Bank Stock Policy [Policy Text Block] federal home loan bank stock. Loans Held For Sale Finance, Loan and Lease Receivables, Held-for-sale, Policy [Policy Text Block] Loans Finance, Loans and Leases Receivable, Policy [Policy Text Block] Interest and Fees on Loans Interest And Fees On Loans Policy [Policy Text Block] Interest and fees on loans. Allowance for Loan Losses Loans and Leases Receivable, Allowance for Loan Losses Policy [Policy Text Block] Servicing Transfers and Servicing of Financial Assets, Servicing of Financial Assets, Policy [Policy Text Block] Other Real Estate Owned Other Real Estate Owned Valuation Allowance Policy [Policy Text Block] Other real estate owned valuation allowance. Bank-Owned Life Insurance Life Insurance Bank Owned Policy [Policy Text Block] Life insurance bank owned policy. Transfers of Financial Assets Transfers and Servicing of Financial Assets, Transfers of Financial Assets, Policy [Policy Text Block] Premises and Equipment Property, Plant and Equipment, Policy [Policy Text Block] Marketing and Promotions Advertising Costs, Policy [Policy Text Block] Impairment of Long-Lived Assets Other Than Goodwill Impairment or Disposal of Long-Lived Assets, Policy [Policy Text Block] Goodwill Goodwill and Intangible Assets, Goodwill, Policy [Policy Text Block] Income Taxes Income Tax, Policy [Policy Text Block] Investments in Limited Partnerships Equity Method Investments [Policy Text Block] Pension and Other Post-Retirement Benefits Pension and Other Postretirement Plans, Policy [Policy Text Block] Fair Values of Financial Instruments Fair Value of Financial Instruments, Policy [Policy Text Block] Earnings per Common Share Earnings Per Share, Policy [Policy Text Block] Employee Stock Ownership Plan Compensation Related Costs, Policy [Policy Text Block] Share-Based Compensation Share-based Compensation, Option and Incentive Plans Policy [Policy Text Block] Off-balance Sheet Financial Instruments Off Balance Sheet Financial Instruments Policy [Policy Text Block] Off Balance Sheet Financial Instruments Policy [Text Block] Segment Information Segment Reporting, Policy [Policy Text Block] Recent Accounting Pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Condensed Financial Information of Parent Company Only Disclosure [Abstract] Condensed Statements of Condition Condensed Balance Sheet [Table Text Block] Condensed Statements of Net Income Condensed Income Statement [Table Text Block] Condensed Statements of Cash Flows Condensed Cash Flow Statement [Table Text Block] Schedule of Accounts, Notes, Loans and Financing Receivable [Table] Schedule of Accounts, Notes, Loans and Financing Receivable [Table] 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] Accounts, Notes, Loans and Financing Receivable [Line Items] Accounts, Notes, Loans and Financing Receivable [Line Items] Total loans Loans and Leases Receivable, Gross 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 Loans and Leases Receivable, Net Amount Derivative Instruments and Hedging Activities Disclosure [Abstract]