Amendment Flag Current Fiscal Year End Date Document Period End Date Entity [Domain] Entity Well-known Seasoned Issuer Entity Voluntary Filers Entity Current Reporting Status Entity Filer Category Entity Public Float Entity Registrant Name Entity Central Index Key Entity Common Stock, Shares Outstanding Document Fiscal Year Focus Document Fiscal Period Focus Legal Entity [Axis] Document Type Entity Shell Company Entity Emerging Growth Company Entity Ex Transition Period Entity Small Business Amortized Cost and Estimated Fair Value of Investment Securities Investment [Table Text Block] Derivative, notional amount ASU 2017-07 [Member] Accounting Standards Update 2017-07 [Member] ACCOUNTING POLICIES [Abstract] ASU 2016-01 [Member] Receivable Type [Axis] Net unrealized (loss) gain on available-for-sale securities Accumulated Other Comprehensive Income (Loss), Debt Securities, Available-for-sale, Adjustment, after Tax Net unrealized loss for pension and other post-retirement obligations Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax Accumulated Other Comprehensive Loss [Member] Accumulated other comprehensive loss Accumulated other comprehensive loss Accumulated Other Comprehensive Income (Loss), Net of Tax Accumulated Other Comprehensive Loss By Component: Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract] Additional paid-in capital Additional Paid-In Capital [Member] Adjustments to reconcile net income to net cash provided by operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Adjustments for New Accounting Pronouncements [Axis] Stock-based compensation Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition All Other [Member] Other Segments [Member] Recoveries Allowance for loan losses [Roll Forward] Charge-offs Allowance for Loan and Lease Losses, Write-offs Allowance for Loan Losses by Class Amortization of mortgage servicing rights Amortization of intangible assets Amortization of intangible assets Amortization of intangible assets Income attributable to unvested stock-based compensation awards Dilutive Securities, Effect on Basic Earnings Per Share, Options and Restrictive Stock Units Weighted-average anti-dilutive stock options outstanding (in shares) Assets: Assets: Assets [Abstract] Total assets Assets Gross unrealized gains Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax Gross unrealized losses Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax Available-for-sale investment securities (cost of $2,957,819 and $3,007,148, respectively) Fair value Available-for-sale investment securities Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract] Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] Amortized cost Available-for-sale Equity Securities, Amortized Cost Basis Gross unrealized losses Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax Available-for-sale investment securities, cost Amortized cost Gross unrealized gains Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax Available-for-Sale Portfolio in Unrealized Loss Positions, Number of Positions [Abstract] Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Number of Positions [Abstract] Available-for-Sale, Debt Maturities, Fair Value [Abstract] Available-for-Sale, Debt Maturities, Amortized Cost [Abstract] Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] Gross unrealized losses Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax Subtotal Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Amortized Cost Available-for-Sale Portfolio [Abstract] Available-for-sale investment securities [Abstract] Gross unrealized gains Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax Subtotal Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Fair Value Amortized cost Debt Securities, Available-for-sale, Amortized Cost Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] Due after five years through ten years Due in one year or less Due after ten years Due after ten years Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value Due after five years through ten years Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value Without single maturity date Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value Due after one through five years Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value Fair value Fair value Available-for-sale investment securities Debt Securities, Available-for-sale Due after one through five years Without single maturity date Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost Fair value Marketable equity securities Available-for-sale Securities, Equity Securities Due in one year or less Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value Balance Sheet Location [Domain] Balance Sheet Location [Axis] Income from bank-owned life insurance policies Bank Owned Life Insurance Income Banking [Member] Banking [Member] Basis of Presentation Accrued interest receivable Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables Investment securities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Marketable Securities Other assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets Community Bank System, Inc. common stock Assets acquired Long-term debt Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt Cash and cash equivalents Purchase price of acquisition Total net consideration paid Deferred tax liability Other liabilities Other liabilities Business Acquisition [Line Items] Business Acquisition, Acquiree [Domain] Issuance of common shares (in shares) Business Acquisition [Axis] ACQUISITIONS [Abstract] ACQUISITIONS Business Combination Disclosure [Text Block] Business development and marketing Total identifiable assets, net Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net Consideration paid [Abstract] Premises and equipment Intangibles Acquisition expenses Acquisition expenses Merger and acquisition integration-related expenses Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Less Noncontrolling Interest [Abstract] Acquisitions [Abstract] Estimated fair value of assets and liabilities assumed in acquisition [Abstract] Carrying Value [Member] Change in cash and cash equivalents Cash and Cash Equivalents, Period Increase (Decrease) Cash acquired from acquisition Cash and cash equivalents at end of period Cash and cash equivalents at beginning of period Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Supplemental disclosures of noncash financing and investing activities: Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Summary of loans acquired [Abstract] Fair value of acquired loans Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value Accretion recognized, year-to-date Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield, Accretion Net reclassification between accretable and non-accretable Beginning Balance Ending Balance Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Accretable discount related to credit impaired acquired loans [Roll forward] Contractually required principal and interest at acquisition Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, Contractually Required Payments Receivable at Acquisition Expected cash flows at acquisition Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, Cash Flows Expected to be Collected at Acquisition Certificates of Deposit [Member] Commercial Portfolio Segment [Member] Business Lending [Member] Commercial Loan [Member] COMMITMENTS, CONTINGENT LIABILITIES AND RESTRICTIONS Commitments and Contingencies Disclosure [Text Block] Commitments to Extend Credit [Member] COMMITMENTS, CONTINGENT LIABILITIES AND RESTRICTIONS [Abstract] Commitments and contingencies (See Note J) Commitments and Contingencies Common Stock [Member] Common stock, shares authorized (in shares) Cash dividends declared per share (in dollars per share) Dividends declared per common share (in dollars per share) Common stock, par value (in dollars per share) Common stock, $1.00 par value, 75,000,000 shares authorized; 51,547,253 and 51,263,841 shares issued, respectively Common stock, shares issued (in shares) Balance (in shares) Balance (in shares) Common Stock, Shares, Outstanding Data processing and communications Comprehensive income Comprehensive Income (Loss), Net of Tax, Attributable to Parent Consumer Portfolio Segment [Member] Contract Balances [Abstract] Contract with Customer, Asset and Liability [Abstract] Accounts receivable Contract with Customer, Asset, Net Core Deposit Intangibles [Member] Core Deposits [Member] Corporate Debt Securities [Member] Credit Loss Status [Domain] Credit Loss Status [Axis] Cumulative effect of change in accounting principle - equity securities Reclassification of other comprehensive income to retained earnings Customer Lists [Member] Doubtful [Member] Designated as Hedging Instrument [Member] 12 months or longer Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss Less than 12 months Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months Less than 12 months Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions 12 months or longer Less than 12 months Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss Total Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions Total Debt Securities, Available-for-sale, Unrealized Loss Position Total Debt Securities, 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Fair value of derivate assets Derivative asset Commitments to originate real estate loans for sale Derivative [Line Items] DERIVATIVE INSTRUMENTS Derivative Instruments and Hedging Activities Disclosure [Text Block] Derivative Instruments [Abstract] Derivative Instrument Detail [Abstract] Derivative Instrument [Axis] Derivative [Table] DERIVATIVE INSTRUMENTS [Abstract] Derivative liability Derivative Liability Derivative weighted average pay rate Derivative, Fixed Interest Rate Fair value of derivate liability Hedging Relationship [Axis] Derivative Contract [Domain] Derivative weighted average receive rate Derivative, Variable Interest Rate Common, $1.06 per share Dividends, Common Stock Cash dividends declared: Dividends [Abstract] Dividends declared and unpaid Reconciliation of basic to diluted earnings per share [Abstract] Earnings Per Share Reconciliation [Abstract] EARNINGS PER SHARE Earnings Per Share [Text Block] Basic earnings per share (in dollars per share) Earnings Per Share, Basic Basic earnings per share [Abstract] Earnings Per Share, Basic [Abstract] Diluted earnings per share (in dollars per share) Earnings Per Share, Diluted Diluted earnings per share [Abstract] Earnings Per Share, Diluted [Abstract] EARNINGS PER SHARE [Abstract] Unrealized gain on equity securities Unrealized gain on equity securities Unrealized gain on equity securities Equity Securities without Readily Determinable Fair Value, Upward Price Adjustment, Annual Amount Amortized cost Equity Securities, FV-NI, Cost Equity Component [Domain] Fair value Equity securities Equity Securities, FV-NI Percent ownership of unconsolidated subsidiary trusts Fair Value [Member] Withholding taxes paid on share-based compensation Total Fair Value, Net Asset (Liability) Assets and Liabilities Measured on Nonrecurring Basis [Abstract] Fair Value Measurements, Nonrecurring Value Measurement [Abstract] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Fair 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[Line Items] Carrying Amounts and Estimated Fair Values of Other Financial Instruments Significant Unobservable Inputs, Fair Value Valuation Techniques Fair Value, Assets and Liabilities Measured on Nonrecurring Basis, Valuation Techniques [Table Text Block] Fair Value, by Balance Sheet Grouping [Table] Fair Value Measurement [Domain] Level 3 [Member] Level 1 [Member] Level 2 [Member] Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items] Subordinated debt held by unconsolidated subsidiary trusts Fair Value Hedging [Member] Acquired Impaired [Member] Financial Asset Acquired with Credit Deterioration [Member] Financial Instrument Performance Status [Domain] Financial Instrument Performance Status [Axis] Financial Instrument [Axis] Financial liabilities [Abstract] Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract] Financial assets [Abstract] Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] Total past due Loans by Credit Quality Indicator Financing Receivable Credit Quality Indicators [Table Text Block] Class of Financing Receivable, Type [Domain] Nonaccrual Financing Receivable, Allowance for Credit Losses [Line Items] Loans modified in TDR during the year, number Financing Receivable, Modifications [Line Items] Financing Receivables, Period Past Due [Axis] Financing Receivable Portfolio Segment [Axis] Current Financing Receivables, Period Past Due [Domain] Loans modified in TDR during the year, amount Financing Receivable, Recorded Investment [Line Items] Class of Financing Receivable, Type [Axis] Aged analysis of the company's loans [Abstract] Financing Receivable Portfolio Segment [Domain] TDRs, amount 90 + days past due and still accruing 2019 Finite-Lived Intangible Assets, Amortization Expense, Year Two 2022 Finite-Lived Intangible Assets, Amortization Expense, Year Five Oct - Dec 2018 Net carrying amount Finite-Lived Intangible Assets, Net Gross carrying amount 2020 Finite-Lived 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Balance Specifically allocated allowance Valuation allowance Summary of Non-Business Impaired Loans Impaired Financing Receivables [Table Text Block] Loans with allowance allocation Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance Income before income taxes Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest CONSOLIDATED STATEMENTS OF INCOME (Unaudited) [Abstract] Income taxes Income Tax Expense (Benefit) Cash paid for income taxes Net increase in deposits Increase in deferred compensation arrangements Change in other assets and other liabilities Increase (Decrease) in Other Operating Assets and Liabilities, Net Increase (Decrease) in Stockholders' Equity [Roll Forward] Assumed exercise of stock options (in shares) Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements Insurance services Insurance Services Revenue Total interest income Interest and 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Classified by Contractual Maturity Date [Table Text Block] INVESTMENT SECURITIES [Abstract] INVESTMENT SECURITIES Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] Terms of preferred securities for each trust [Abstract] Junior Subordinated Debenture Owed to Unconsolidated Subsidiary Trust [Abstract] Subordinated debt held by unconsolidated subsidiary trusts Junior Subordinated Debenture Owed to Unconsolidated Subsidiary Trust LIBOR [Member] Long-term Debt, Type [Domain] Long-term Debt, Type [Axis] Salaries and employee benefits Fair value of liabilities assumed Total liabilities Liabilities Total liabilities and shareholders' equity Liabilities and Equity Increase (decrease) in other liabilities as a result of reclassification and adjustment Liabilities, Fair Value Adjustment Liabilities: Liabilities [Abstract] Commitments to Originate Real Estate Loans for Sale [Member] Loan Origination Commitments [Member] Impaired loans [Abstract] Loans and Leases Receivable Disclosure [Table] Loans and Leases Receivable Disclosure [Line Items] Allowance for loan losses Ending balance Beginning balance Loans and Leases Receivable, Allowance Business Lending Loans by Credit Quality Category [Abstract] Loans and Leases Receivable, before Fees, Gross [Abstract] Loans held for sale, at fair value Loans Receivable Held-for-sale, Net, Not Part of Disposal Group, Mortgage Loans Gross loans, including deferred origination costs Total loans Loans and Leases Receivable, Gross Net loans Loans and Leases Receivable, Net Amount LOANS [Abstract] Net loans LOANS Loans, Notes, Trade and Other Receivables Disclosure [Text Block] Mandatorily Redeemable Preferred Securities [Abstract] Long-term Debt, by Type, Current and Noncurrent [Abstract] Commitments, Contingent Liabilities and Restrictions [Abstract] Loss Contingency, Estimate [Abstract] Range of reasonably possible losses for such matters in the aggregate, beyond the existing recorded liability Loss Contingency, Estimate of Possible Loss us-gaap_MarketApproachValuationTechniqueMember Investment Securities Gain on sales of investment securities us-gaap_MeasurementInputCostToSellMember Government Agency Mortgage-Backed Securities [Member] Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] Mortgage loans held for sale Mortgages Held-for-sale, Fair Value Disclosure Nonqualified Plan [Member] Nonperforming [Member] Nonperforming Financial Instruments [Member] Financing activities: Net cash used in financing activities Net Cash Provided by (Used in) Financing Activities Net cash provided by investing activities Net Cash Provided by (Used in) Investing Activities Investing activities: Operating activities: Income available to common shareholders Net Income (Loss) Available to Common Stockholders, Basic Net cash provided by operating activities Net Cash Provided by (Used in) Operating Activities Net income Net income New Accounting Pronouncements New Accounting Pronouncements or Change in Accounting Principle [Line Items] Recently Adopted Accounting Pronouncements [Abstract] New Accounting Pronouncements or Change in Accounting Principle [Table] Acquisitions: Total noninterest expenses Noninterest Expense Total noninterest revenues Noninterest revenues Noninterest Income Noninterest expenses: Noninterest Expense [Abstract] Noninterest-bearing deposits Noninterest revenues: Loans receivable, net [Abstract] Occupancy and equipment Operating Segments [Member] Expenses incurred BASIS OF PRESENTATION Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] BASIS OF PRESENTATION [Abstract] Other real estate owned, Measurement Input [Extensible List] Other real estate owned, measurement input Other real estate owned, Valuation Technique [Extensible List] Tax effect Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from 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[Member] Other assets Other Assets Other operating expenses Net unrealized holding (losses)/gains arising during period, gross Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, before Tax Net unrealized holding (losses)/gains arising during period, net Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax Tax effect Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax Other long-term debt Amortized cost Other Investments and Securities, at Cost Post-retirement Benefits [Member] Other Postretirement Benefits Plan [Member] Other expenses Other Noninterest Expense Other real estate owned Other real estate owned Other Liabilities [Member] Other Assets [Member] Unrealized (losses) gains on available-for-sale securities: Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax, Portion Attributable to Parent [Abstract] Pension and other post retirement obligations: Other comprehensive (loss)/income related to unrealized (losses)/gains on available-for-sale securities, net of taxes Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax, Portion Attributable to Parent Other comprehensive (loss)/income, net of tax Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent Other comprehensive (loss) income related to pension and other post retirement obligations, net of taxes Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax and Reclassification Adjustment, Attributable to Parent Performing [Member] Performing Financial Instruments [Member] Aged Analysis of Past Due Loans by Class Pass [Member] Net (increase) decrease in loans Payments for (Proceeds from) Loans and Leases Purchases of treasury stock Payments for Repurchase of Common Stock Cash dividends paid 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Recognition Revenue from Contract with Customer [Policy Text Block] Revenues earned Revenues Classified [Member] Substandard [Member] Estimated Aggregate Amortization Expense for Each of Five Succeeding Fiscal Years Schedule of Finite-Lived Intangible Assets, Future Amortization Expense [Table Text Block] Schedule of Financing Receivables Past Due [Table] Schedule of Financing Receivable, Recorded Investment, Credit Quality Indicator [Table] Summary of Investment Securities That Have Been in a Continuous Unrealized Loss Position for Less Than or Greater Than Twelve Months Schedule of Unrealized Loss on Investments [Table Text Block] Net Periodic Benefit Cost Schedule of Net Benefit Costs [Table Text Block] Reconciliation of Basic to Diluted Earnings per Share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Gross Carrying Amount and Accumulated Amortization of Identifiable Intangible Assets Schedule of Finite-Lived Intangible Assets [Table Text Block] Schedule of Financing Receivable, Troubled Debt Restructurings [Table] Estimated Fair Value of Assets Acquired and Liabilities Assumed Schedule of Finite-Lived Intangible Assets [Table] Fair Value Measured on a Recurring Basis Schedule of Financing Receivable, Allowance for Credit Losses [Table] Schedule of Available-for-sale Securities [Line Items] Schedule of Available-for-sale Securities [Table] Schedule of Defined Benefit Plans Disclosures [Table] Schedule of Business Acquisitions, by Acquisition [Table] Schedule of Fair Value, Off-balance Sheet Risks [Table] Components of Goodwill Schedule of Goodwill [Table Text Block] Schedule of Segment Reporting Information, by Segment [Table] Segment Reporting Information by Segment Loans Receivable, Net Schedule of Accounts, Notes, Loans and Financing Receivable [Table Text Block] Securities sold under agreement to repurchase, short-term Securities Sold under Agreements to Repurchase Securities sold under agreement to repurchase, short-term Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure Information about reportable segments [Abstract] Segment Reporting Information [Line Items] SEGMENT INFORMATION Segment Reporting Disclosure [Text Block] SEGMENT INFORMATION [Abstract] Segments [Domain] Preferred Debt [Member] Senior Notes [Member] Stock-based compensation Short-term borrowings ACCOUNTING POLICIES Significant Accounting Policies [Text Block] Special Mention [Member] Standby Letters of Credit [Member] CONSOLIDATED STATEMENTS OF CONDITION (Unaudited) [Abstract] CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) [Abstract] Segments [Axis] Statement [Line Items] Statement [Table] CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) [Abstract] Equity Components [Axis] CONSOLIDATED STATEMENT OF CHANGES IN SHAREHOLDERS' EQUITY (Unaudited) [Abstract] Common stock issued under employee stock ownership plan (in shares) Stock Issued During Period, Shares, Employee Stock Ownership Plan Common stock issued under employee stock ownership plan Stock Issued During Period, Value, Employee Stock Ownership Plan Treasury stock issued to benefit plans, net (in shares) Stock Issued During Period, Shares, Treasury Stock Reissued Common stock issued Stock Issued Number of common shares authorized to be repurchased (in shares) Treasury stock issued to benefit plans, net Total shareholders' equity Balance Balance Stockholders' Equity Attributable to Parent Shareholders' equity: Shareholders' equity: Stockholders' Equity Attributable to Parent [Abstract] Subsequent Event Type [Domain] Subsequent Event Type [Axis] Subsequent Event [Member] Supplemental disclosures of cash flow information: Supplemental Cash Flow Elements [Abstract] Transfers from loans to other real estate Real Estate Owned, Transfer to Real Estate Owned Financial Instruments [Domain] Number of common shares repurchased (in shares) Treasury Stock [Member] Treasury stock, at cost (410,577 shares including 206,151 shares held by deferred compensation arrangements at September 30, 2018, and 567,764 shares including 237,494 shares held by deferred compensation arrangements at December 31, 2017, respectively) Treasury Stock, Value Treasury stock, shares at cost (in shares) Troubled Debt Restructurings on Financing Receivables Type of Adoption [Domain] Unfunded Plan [Member] Unallocated Financing Receivables [Member] Unbilled fee revenue Unbilled Contracts Receivable Obligations of State and Political Subdivisions [Member] US States and Political Subdivisions Debt Securities [Member] U.S. Treasury Securities [Member] U.S. Treasury and Agency Securities [Member] us-gaap_ValuationTechniqueDiscountedCashFlowMember Variable Rate [Axis] Variable Rate [Domain] Weighted-average common shares outstanding - basic (in shares) Weighted Average Number of Shares Outstanding, Basic Weighted-average common shares outstanding - diluted (in shares) Weighted Average Number of Shares Outstanding, Diluted Consolidation Items [Axis] Consolidation Items [Domain] Maximum [Member] Minimum [Member] Range [Domain] Range [Axis] Geographical [Domain] Geographical [Axis] Weighted Average [Member] Massachusetts [Member] MASSACHUSETTS VERMONT [Member] The gains (losses) included in earnings that represent the difference between the sale price and the carrying value of loans and leases and other assets that were sold during the reporting period. This element refers to the gain (loss) and not to the cash proceeds of the sales. This element is a adjustment to net income when calculating net cash generated by operating activities using the indirect method. Net gain (loss) on sale of loans and other assets Net (gain) loss on sale of loans and other assets The sum of periodic adjustments charged to earnings related to investment securities, loans and borrowings. As a noncash item this element is an adjustment to net income when calculating cash provided by or used in operations using the indirect method. Accretion Amortization on Investments Loans and Borrowings Net accretion on securities, loans and borrowings The cash inflow from settlement of bank-owned life insurance policies. Settlement of Bank-owned Life Insurance Policies Settlement of bank-owned life insurance policies Tabular disclosure of the nature and terms of the financial instruments with off-balance sheet risk. Off Balance Sheet Financial Instruments [Table Text Block] Off Balance Sheet Financial Instruments Contract Amounts Fair value of other investments and equity securities. Other Investment and Equity Securities, Fair Value Equity and other securities (cost of $42,281 and $50,291, respectively) Fair value The carrying amount, net of accumulated amortization as of the balance sheet date of intangible assets not otherwise specified in the taxonomy having a reasonably expected period of economic benefit. Other Finite Lived Intangible Assets Net Other intangibles, net Intangible assets, net of accumulated amortization, related to bank deposits belonging to an acquired bank that can be counted on as a stable source of lending funds, net of accumulated amortization. These deposits have the following attributes: predictable costs and low sensitivity to interest rates. This intangible asset is usually amortized over its estimated useful life. Core Deposit Intangibles Net Core deposit intangibles, net Accrued interest on deposits and other liabilities as of the date of the balance sheet of the reporting entity. Accrued Interest and Other Liabilities Accrued interest and other liabilities The aggregate sum of net carrying value of a all finite-lived intangible asset classes and goodwill, less accumulated amortization and any impairment charges. Intangible assets, net Intangible assets, net Cost of other investments and equity securities that are not at fair value as of the balance sheet date and those that are not accounted for under the equity method. Other Investments and Equity Securities, at Cost Equity and other securities, cost Amortized cost MANDATORILY REDEEMABLE PREFERRED SECURITIES [Abstract] The entire disclosure for mandatorily redeemable preferred securities due to sponsorship of business trusts. MANDATORILY REDEEMABLE PREFERRED SECURITIES [Text Block] MANDATORILY REDEEMABLE PREFERRED SECURITIES Represents a business trust that was formed for the purpose of issuing company-obligated mandatorily redeemable preferred securities to third-party investors and investing the proceeds from the sale of such preferred securities solely in junior subordinated debt securities of the Company. Community Statutory Trust 3 [Member] Community Statutory Trust III [Member] Represents a business trust that was formed for the purpose of issuing company-obligated mandatorily redeemable preferred securities to third-party investors and investing the proceeds from the sale of such preferred securities solely in junior subordinated debt securities of the Company. MBVT Statutory Trust I [Member] Term of the interest rate that fluctuates over time as a result of an underlying benchmark interest rate or index, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Debt Instrument Term Of Variable Rate Term of variable rate The cash inflow from the exercise of the rights to redeem debentures and trust preferred securities, net of the cash outflow for repurchasing trust preferred securities, by a business trust or other special purpose entity, mainly established by a bank holding entity, to third party investors. The trust's assets are deeply subordinated debentures of the bank holding entity. Most trust preferred securities are subject to a mandatory redemption upon the repayment of the debentures. Proceeds From Redemption of Debentures and Preferred Securities Redemption of debentures and associated preferred securities The number of unconsolidated subsidiary trust created for the issuance of debt. Such debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets and was initially scheduled to be repaid after one year or beyond the normal operating cycle if longer. Number of Unconsolidated Subsidiary Business Trusts Owned Number of unconsolidated subsidiary business trusts owned Represents a business trust that was formed for the purpose of issuing company-obligated mandatorily redeemable preferred securities to third-party investors and investing the proceeds from the sale of such preferred securities solely in junior subordinated debt securities of the Company. Community Statutory Trust 4 [Member] Community Capital Trust IV [Member] Tabular disclosure of information pertaining to subordinated long-term debt owed to unconsolidated subsidiary trusts, including but not limited to identification of terms, features, collateral requirements and other information necessary to a fair presentation. Junior Subordinated Debenture Owed To Unconsolidated Subsidiary Trust [Table Text Block] Terms of Preferred Securities Stock Repurchase Program [Abstract] Stock Repurchase Program [Abstract] The category includes multiclass, pay-through securitizations collateralized by mortgages secured by assets and guaranteed by a US agency, such as automobiles and boats, which are generally structured so that the collections of principal and interest due from the underlying debtors are paid to the holders of the CMO. Government Agency Collateralized Mortgage Obligations [Member] Equity Securities, FV-NI [Abstract] Equity Securities, at Fair Value [Abstract] Other Securities [Abstract] Other Securities [Abstract] Equity and Other Securities [Abstract] Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in equity and other securities. Other Investments and Equity Securities, Accumulated Gross Unrealized Gain, before Tax Gross unrealized gains Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in other investments and securities. Other Investments and Securities, Accumulated Gross Unrealized Gain Before Tax Gross unrealized gains Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in equity securities. Equity Securities, FV-NI, Accumulated Gross Unrealized Loss Before Tax Gross unrealized losses Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in equity securities. Equity Securities, FV-NI, Accumulated Gross Unrealized Gain Before Tax Gross unrealized gains Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in other investments and equity securities. Other Investments and Equity Securities, Accumulated Gross Unrealized Loss, before Tax Gross unrealized losses Fair value of other investments and securities. Other Investments and Securities, Fair Value Fair value This category includes information about investments in equity securities which are not otherwise provided for in the existing taxonomy in the form of Domain Members or individual elements. Other Equity Securities [Member] Federal reserve bank stock. Federal Reserve Bank Stock [Member] Federal Reserve Bank Common Stock [Member] Amount after tax of other comprehensive income (loss) attributable to parent entity, before cumulative effect of change in accounting principle. Other Comprehensive Income (Loss), Net of Tax, Before Cumulative Effect of Change in Accounting Principle, Portion Attributable to Parent Other comprehensive loss, net of tax Aggregate value of stock issued during the period as a result of distribution of stock under deferred compensation arrangements. Stock Issued During Period, Value, Deferred Compensation Arrangements Distribution of stock under deferred compensation arrangements Number of shares of stock issued in connection with distribution of stock under deferred compensation arrangements. Stock Issued During Period, Shares, Deferred Compensation Arrangements Distribution of stock under deferred compensation arrangements (in shares) Equity Securities, Available-for-sale, Fair Value to Amortized Cost Basis [Abstract] Marketable Equity Securities [Abstract] Amount of Business Lending Loans by Credit Quality Category [Abstract] Loans acquired after January 1, 2009 that have evidence of deterioration in credit quality since origination and for which it is probable, at acquisition, that the company will be unable to collect all contractually required payments. Purchased Impaired [Member] Acquired Impaired [Member] Includes all types of loans, except for business lending loans. All Other Loans [Member] A revolving, open-end loan extended under a line of credit or installment loans and secured by the borrower's residential property. Home Equity Receivables [Member] Home Equity [Member] Amounts receivable from issuing a installment loans to individuals that allows someone to make a purchase on borrowed money. Consumer Installment Direct [Member] Consumer Direct [Member] Amounts receivable from issuing a installment loans to individuals through dealerships and secured by auto, marine or other recreational vehicles. Consumer Installment Indirect [Member] Consumer Indirect [Member] Loans purchased or acquired in a business combination after January 1, 2009. Acquired Loans [Member] Loans not purchased or acquired in a business combination after January 1, 2009. Legacy Loan [Member] Legacy Loans [Member] Accumulated appreciation or loss, in value of the total of available-for-sale securities at the end of an accounting period, before tax. Accumulated Other Comprehensive Income (Loss), Available For Sale Securities Adjustment, Gross Unrealized (loss) gain on available-for-sale securities The tax effect of gross gain (loss), prior service cost (credit), and transition assets (obligations), as well as minimum pension liability if still remaining, included in accumulated other comprehensive income associated with a defined benefit pension or other postretirement plan(s) because they have yet to be recognized as components of net periodic benefit cost. Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Tax Effect Tax effect The gross gain (loss), prior service cost (credit), and transition assets (obligations), as well as minimum pension liability if still remaining, included in accumulated other comprehensive income associated with a defined benefit pension or other postretirement plan(s) because they have yet to be recognized as components of net periodic benefit cost. Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Gross of Tax Unrealized loss for pension and other post-retirement obligations The tax effect of accumulated appreciation or loss in value of the total of available-for-sale securities at the end of an accounting period. Accumulated Other Comprehensive Income (Loss), Available For Sale Securities Adjustment, Tax Effect Tax effect Amount, after tax, of gain (loss) for (increase) decrease in value of benefit obligation for change in actuarial assumptions and increase (decrease) in value of plan assets from experience different from that assumed of defined benefit plan due to plan merger, that has not been recognized in net periodic benefit (cost) credit. Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period Due to Plan Merger, after Tax Unamortized actuarial gain due to plan merger, net Amount, before tax, of gain (loss) for (increase) decrease in value of benefit obligation for change in actuarial assumptions and increase (decrease) in value of plan assets from experience different from that assumed of defined benefit plan due to plan merger, that has not been recognized in net periodic benefit (cost) credit. Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period Due to Plan Merger, before Tax Unamortized actuarial gain due to plan merger, gross (See Note H) Amount of tax expense (benefit) for (increase) decrease in value of benefit obligation for change in actuarial assumptions and increase (decrease) in value of plan assets from experience different from that assumed of defined benefit plan due to plan merger, that has not been recognized in net periodic benefit (cost) credit. Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period Due to Plan Merger, Tax Tax effect Amount after tax of reclassification adjustment from accumulated other comprehensive income due to change in accounting principle for equity securities. Accumulated Other Comprehensive Income, Reclassification Adjustment for Equity Securities, Net of Tax Reclassification of other comprehensive income due to change in accounting principle - equity securities Amount after tax of reclassification adjustment from accumulated other comprehensive income due to change in accounting principle for equity securities. Other Comprehensive Income Reclassification Adjustment For Equity Securities Net Of Tax Reclassification of other comprehensive income due to change in accounting principle - equity securities Amount, before tax, of reclassification adjustment from accumulated other comprehensive (income) loss for initial projected benefit obligation recognized upon adoption of defined benefit plan. Other Comprehensive Income Loss Amortization Adjustment from AOCI Pension and Other Postretirement Benefit Plans for Initial Projected Benefit Obligation Recognized Upon Plan Adoption Before Tax Initial projected benefit obligation recognized upon plan adoption, gross (See Note H) Amount of tax (expense) benefit of reclassification adjustment from accumulated other comprehensive (income) loss for initial projected benefit obligation recognized upon adoption of defined benefit plan. Other Comprehensive Income Loss Amortization Adjustment from AOCI Pension and Other Postretirement Benefit Plans for Initial Projected Benefit Obligation Recognized Upon Plan Adoption Tax Tax effect Amount, after tax, of reclassification adjustment from accumulated other comprehensive (income) loss for initial projected benefit obligation recognized upon adoption of defined benefit plan. Other Comprehensive Income Loss Amortization Adjustment from AOCI Pension and Other Postretirement Benefit Plans For Initial Projected Benefit Obligation Recognized Upon Plan Adoption Net Of Tax Initial projected benefit obligation recognized upon plan adoption, net The typical original contract term of financial receivable, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Financial Receivable, Contract Term Typical contract term The undiscounted sum of all amounts, including amounts deemed principal, interest, fees, penalties, and other under the loan, owed to the investor at the reporting date, whether or not currently due and whether or not any such amounts have been written or charged off by the investor. This relates to loans not accounted for as debt securities, with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable. Certain Loans Acquired In Transfer Not Accounted For As Debt Securities Impairment Outstanding Balance Credit impaired acquired loans, total balance due Revenues pertaining to bank-related activities not specified elsewhere. Other banking services Other banking services Revenues related to employee benefit services. Employee Benefit Services Employee benefit services The non-interest income earned from account activity fees, monthly service fees, check orders, debit and credit card income, ATM fees, Merchant services income and other revenues from processing wire transfers, bill pay service, cashier's checks and foreign exchange. Deposit Service Fees Deposit service fees Fee income charged during the period for wealth management services. Wealth Management Services Wealth management services Tabular disclosure of changes to the aggregate cash flows related to loans expected to be collected over the investor's initial investment in the loan. This relates to loans with deteriorating of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable. Schedule of accretable discount related to credit impaired acquired loans [Table Text Block] Accretable Discount Related to Credit Impaired Acquired Loans Principal amount of mortgage loans held for sale. Mortgage loans held for sale, at principal value Mortgage loans held for sale, at principal value Fair value portion of long-term debt classified as other. Other Long-term Debt, Fair Value Disclosure Other long-term debt Fair value portion of debt having initial terms less than one year or the normal operating cycle, if longer. Short-term Debt, Fair Value Disclosure Short-term borrowings Loans acquired after January 1, 2009 that have evidence of deterioration in credit quality since origination and for which it is probable, at acquisition, that the company will be unable to collect all contractually required payments. Financing Receivable, Recorded Investment, Purchased Impaired Acquired impaired Financing receivables that are more than 29 days past due but fewer than 90 days past due. Financing Receivables, 30 to 89 Days Past Due [Member] Past Due 30 - 89 Days [Member] The employee benefit services reportable operating business segment. Employee Benefit Services [Member] Troubled Debt Restructurings [Abstract] Troubled Debt Restructurings (TDRs) [Abstract] The threshold balance of financing receivable that is individually evaluated for impairment. Financing Receivable, Individually Evaluated for Impairment, Threshold Balance Threshold balance of loans individually evaluated for impairment Represents accruing TDRs. Accruing TDRs [Member] Accruing [Member] Represents nonaccrual TDRs. Nonaccrual TDRs [Member] Nonaccrual [Member] Threshold balance of financing receivables classified as a troubled debt restructuring that are collectively evaluated for impairment. Financing Receivable, Modifications, Collectively Evaluated For Impairment, Threshold Balance Threshold balance of TDR loans collectively included in general loan loss allocation and qualitative review Number of financing receivables that have been modified by troubled debt restructurings. Financing Receivable, Modifications, Number of Contracts TDRs, number Capitalized servicing rights, which contractually entitle the servicer to receive fees and ancillary revenues for performing billing, collection, disbursement and recordkeeping services in connection with a mortgage portfolio. Mortgage Servicing Rights [Member] Mortgage Servicing Rights [Member] Amount of contractual balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses. Impaired Financing Receivable, Contractual Balance Contractual balance Document and Entity Information [Abstract] An insurance agency headquartered in Oneonta, New York. Gordon B. Roberts Agency, Inc [Member] Gordon B. Roberts Agency, Inc [Member] A financial services business headquartered in Canton, New York. Styles Bridges Associates [Member] Styles Bridges Associates [Member] A benefits consulting group headquartered in Forest Hills, New York, which was acquired by the Company, through its subsidiary, OneGroup on January 1, 2017. Benefits Advisory Service, Inc [Member] BAS [Member] Parent company of Merchants Bank headquartered in South Burlington, Vermont. Merchants Bancshares, Inc. [Member] Merchants Bancshares, Inc. [Member] A benefits consulting group headquartered in Puerto Rico. HR Consultants (SA), LLC [Member] An insurance agency headquartered in Johnson City, New York. Penna & Associates Agency, Inc. [Member] Penna & Associates Agency, Inc. [Member] A financial services business headquartered in Wilkes Barre, Pennsylvania. Northeast Capital Management, Inc [Member] Northeast Capital Management, Inc [Member] An insurance agency headquartered in Hazleton, Pennsylvania, which was acquired by the Company through its subsidiary, OneGroup NY, Inc. ("OneGroup") on March 1, 2017. Dryfoos Insurance Agency, Inc [Member] Dryfoos [Member] Refers to the number of branch locations added upon merger. Number of Branch Locations Added upon Merger Number of branch locations added upon merger The amount of identifiable net tangible assets acquired at the acquisition date. Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Tangible Assets, Net Tangible assets, net A company headquartered in Woburn, Massachusetts, which was acquired on February 3, 2017. NRS was a privately held corporation focused on providing institutional transfer agency, master recordkeeping services, custom target date fund administration, trust product administration and customized reporting services to institutional clients. Northeast Retirement Services, Inc. [Member] NRS [Member] Refers to the factor of straight-line rate of depreciation used in declining balance depreciation method. Declining Balance Depreciation Method, Factor of Straight-line Rate of Depreciation Factor of straight-line rate of depreciation used in declining balance depreciation method Tabular disclosure regarding loans acquired during the period. Summary of loans acquired [Table Text Block] Summary of Loans Acquired Investment, Debt and Equity Securities [Abstract] Investment Securities [Abstract] Wealth Management Services [Abstract] Refers to number of types of performance obligations related to wealth management services. Number of Types of Performance Obligations Number of types of performance obligations An insurance agency operating in Canton, New York, doing business as The Clark Insurance Agencies ("WJL"), which was acquired by the Company, through its subsidiary, CBNA Insurance Agency, Inc.("CBNA Insurance") on January 4, 2016. WJL Agencies Inc [Member] Represents the other businesses acquired by the entity during the period. Other Acquisitions [Member] Other [Member] Represents the total business acquired by the entity in 2017. Total Acquisitions in 2017 [Member] Total [Member] Represents the second group of businesses acquired by the entity during the period. Other Acquisitions Two [Member] Other [Member] Amount of subordinated debt held by unconsolidated subsidiary trusts due after one year or the normal operating cycle, if longer, assumed at the acquisition date. Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Subordinated Debt Held By Unconsolidated Subsidiary Trusts Subordinated debt held by unconsolidated subsidiary trusts Amount of short-term advances due within one year or within the normal operating cycle, assumed at the acquisition date. Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Short-term Advances Short-term advances Amount of securities sold under agreement to repurchase due within one year or within the normal operating cycle, assumed at the acquisition date. Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Securities Sold Under Agreement to Repurchase, Short-term Securities sold under agreement to repurchase, short-term Amount of loans acquired in an acquisition. Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Loans Acquired Loans Amount of deposits acquired in an acquisition. Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deposits Acquired Deposits Disclosure of accounting policy pertaining to new accounting pronouncements adopted that impact the entity's financial reporting. Recently Adopted Accounting Pronouncement [Policy Text Block] Recently Adopted Accounting Pronouncements Loans acquired after January 1, 2009 that do not have evidence of deterioration in credit quality since origination and for which it is probable, at acquisition that the company will be unable to collect all contractually required payments. Acquired Non Impaired Loans [Member] Acquired Non-impaired Loans [Member] The acquirer's estimate, at acquisition, of the amount and timing of undiscounted principal, interest, and other cash flows not expected to be collected. Certain Loans Acquired In Transfer Accounted For As Debt Securities Acquired During Period, Contractual Cash Flows Not Expected To Be Collected Contractual cash flows not expected to be collected The investor's estimate, at acquisition, of the amount and timing of the interest component of expected cash flows for loans acquired in transfer not accounted for as debt securities. Certain Loans Acquired In Transfer Not Accounted For As Debt Securities Acquired During Period Interest Component Of Cash Flows Interest component of expected cash flows Loans acquired after January 1, 2009 that have evidence of deterioration in credit quality since origination and for which it is probable, at acquisition that the company will be unable to collect all contractually required payments. Acquired Impaired Loans [Member]