ASU 2016-13 [Member] Topic ASC 326 [Member] ACCOUNTING POLICIES [Abstract] Receivable Type [Axis] Accumulated Other Comprehensive (Loss) Income [Member] Accumulated other comprehensive income (loss) Accumulated other comprehensive gain (loss) Net unrealized gain on available-for-sale securities AOCI, 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 Income By Component: Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract] Additional Paid-In Capital [Member] Additional paid-in capital Stock-based compensation APIC, Share-based Payment Arrangement, Increase for Cost Recognition 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] Accounting Standards Update [Axis] Accounting Standards Update [Axis] All Other [Member] Other Segments [Member] Charge-offs Allowance for Loan and Lease Losses, Write-offs Allowance for credit losses - loans [Roll Forward] Recoveries Increase in allowance for credit losses Allowance for Loan and Lease Losses, Period Increase (Decrease) Steuben acquisition Allowance for Loan and Lease Losses, Loans Acquired Allowance for Credit Losses by Class Allowance for Credit Losses - Debt Securities [Abstract] Allowance for Credit Losses [Abstract] Allowance for Credit Loss [Abstract] 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) Total Assets, Fair Value Disclosure Assets: Assets: Assets [Abstract] Assets Total assets Gross unrealized gains Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax Gross unrealized losses Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, 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 Portfolio [Abstract] Available-for-sale investment securities [Abstract] Available-for-Sale Portfolio, Debt Maturities, Gross Unrealized Losses [Abstract] Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] Amortized cost Debt Securities, Available-for-sale, Amortized Cost Gross unrealized losses Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax Available-for-sale investment securities (cost of $3,129,978 and $3,011,551, respectively) Fair value Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] Gross unrealized gains Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax Subtotal Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Fair Value Available-for-Sale, Debt Maturities, Amortized Cost [Abstract] Available-for-Sale, Debt Maturities, Fair Value [Abstract] Subtotal Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Amortized Cost Due after ten years Debt Securities, Available-for-Sale, Fair Value, Maturity, Allocated and Single Maturity Date, after Year 10 Due after ten years Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] Due after one through five years Debt Securities, Available-for-Sale, Fair Value, Maturity, Allocated and Single Maturity Date, after Year One Through Five Due after five years through ten years Debt Securities, Available-for-Sale, Fair Value, Maturity, Allocated and Single Maturity Date, after Year 5 Through 10 Due after one through five years Available-for-sale investment securities, cost Amortized cost Due after five years through ten years Due in one year or less Fair value Available-for-sale investment securities Fair value Debt Securities, Available-for-sale Without single maturity date Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost Without single maturity date Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value Due in one year or less Debt Securities, Available-for-Sale, Fair Value, Maturity, Allocated and Single Maturity Date, Year One 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 Investment securities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Marketable Securities Community Bank System, Inc. common stock Total net consideration paid Business Combination, Consideration Transferred Other liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other Other assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets Assets acquired Cash and cash equivalents Accrued interest and fees receivable Issuance of common shares (in shares) Business Acquisition, Acquiree [Domain] Total revenue, net of interest expense, pro forma Business Acquisition, Pro Forma Revenue Business Acquisition [Line Items] ACQUISITIONS [Abstract] Business Acquisition [Axis] Net income, pro forma Business Acquisition, Pro Forma Net Income (Loss) Supplemental Pro Forma Financial Information [Abstract] Pro Forma Financial Information Premises and equipment, net Consideration paid [Abstract] Intangibles Total identifiable assets, net Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net ACQUISITIONS Business Combination Disclosure [Text Block] Business development and marketing Acquisition expenses Acquisition expenses Acquisition-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] Estimated fair value of assets and liabilities assumed in acquisition [Abstract] Acquisitions [Abstract] Carrying Value [Member] Cash and cash equivalents at beginning of period Cash and cash equivalents at end of period Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Change in cash and cash equivalents Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect Cash acquired from acquisition 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 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 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 Commercial Portfolio Segment [Member] Business Lending [Member] Commercial Loan [Member] Commitments and contingencies (See Note J) Commitments and Contingencies COMMITMENTS, CONTINGENT LIABILITIES AND RESTRICTIONS Commitments and Contingencies Disclosure [Text Block] Commitments to Extend Credit [Member] COMMITMENTS, CONTINGENT LIABILITIES AND RESTRICTIONS [Abstract] Common Stock Amount Issued [Member] Dividends declared per common share (in dollars per share) Common stock, par value (in dollars per share) Common stock, shares issued (in shares) Common stock, shares authorized (in shares) Common stock, $1.00 par value, 75,000,000 shares authorized; 53,673,335 and 51,974,726 shares issued, respectively Balance (in shares) Balance (in shares) Common Stock, Shares, Outstanding Data processing and communications BENEFIT PLANS [Abstract] Comprehensive income Comprehensive Income (Loss), Net of Tax, Attributable to Parent Consumer Portfolio Segment [Member] Contract Balances [Abstract] Contract with Customer, Contract Asset, Contract Liability, and Receivable [Abstract] Accounts receivable Contract with Customer, Asset, after Allowance for Credit Loss Core Deposit Intangibles [Member] Core Deposits [Member] Corporate Debt Securities [Member] Credit Loss Status [Domain] Credit Score, FICO [Axis] Credit Loss Status [Axis] Credit Score, FICO [Domain] Customer Lists [Member] Doubtful [Member] Designated as Hedging Instrument [Member] Less than 12 months Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions Total Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions Total Debt Securities, Available-for-sale, Unrealized Loss Position 12 months or longer Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Positions Total Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss 12 months or longer Available-for-sale, allowance for credit losses Debt Securities, Available-for-sale, Allowance for Credit Loss Less than 12 months Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months Gross realized losses Gross realized gains Less than 12 months Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 12 months or longer Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss Issuance date Fair Value of Subordinated Notes Payable [Abstract] Debt Instrument, Fair Value Disclosure [Abstract] Variable interest rate, basis spread Par amount Debt Instrument, Face Amount Schedule of Long-term Debt Instruments [Table] Debt Instrument [Axis] Call price Debt Instrument, Call Feature Debt Instrument [Line Items] Debt Instrument, Name [Domain] Fixed interest rate percentage Effective interest rate Debt Instrument, Interest Rate, Effective Percentage Maturity date Maturity date Deferred compensation arrangements (157,897 and 179,548 shares, respectively) Shares held by deferred compensation arrangements (in shares) Deferred Compensation Arrangement with Individual, Common Stock Reserved for Future Issuance Deferred Compensation Arrangements [Member] Pension Plans [Abstract] Defined Benefit Plan, Additional Information [Abstract] Amortization of unrecognized net loss Defined Benefit Plan, Amortization of Gain (Loss) Defined Benefit Plan Disclosure [Line Items] Amortization of prior service cost Expected return on plan assets Defined Benefit Plan, Expected Return (Loss) on Plan Assets Contribution made to defined benefit pension plan by employer Defined Benefit Plan, Plan Assets, Contributions by Employer Net periodic benefit Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Net periodic benefit cost [Abstract] Service cost Interest cost Total deposits Deposits Deposits Deposits, Fair Value Disclosure Depreciation Commitments to originate real estate loans for sale, measurement input Commitments to originate real estate loans for sale, Measurement Input [Extensible List] Derivative Asset, Measurement Input [Extensible List] Weighted average maturity period Commitments to originate real estate loans for sale, Valuation Technique [Extensible List] Derivative Asset, Valuation Technique [Extensible List] Derivative [Line Items] Derivative Instrument [Axis] Derivative Instruments [Abstract] Derivative Instrument Detail [Abstract] Derivative [Table] Commitments to originate real estate loans for sale Derivative asset DERIVATIVE INSTRUMENTS [Abstract] DERIVATIVE INSTRUMENTS Derivative Instruments and Hedging Activities Disclosure [Text Block] Derivative liability Derivative Liability Fair Values of Derivative Instruments [Abstract] Derivative liabilities Derivative assets Derivative Asset, Fair Value, Gross Asset Derivative weighted average pay rate Derivative, Fixed Interest Rate Hedging Relationship [Axis] Derivative, notional amount Derivative weighted average receive rate Derivative, Variable Interest Rate Derivative Contract [Domain] Purchased Credit Deteriorated (PCD) Loans Deteriorated Loans Transferred in, Policy [Policy Text Block] Common 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 [Abstract] Earnings Per Share, Diluted [Abstract] EARNINGS PER SHARE [Abstract] Diluted earnings per share (in dollars per share) Earnings Per Share, Diluted Amortized cost Equity Securities, FV-NI, Cost Unrealized gain (loss) on equity securities Unrealized loss (gain) on equity securities Equity Component [Domain] Fair value Equity securities Equity Securities, FV-NI Equity Securities, at Fair Value [Member] Equity Securities [Member] Fair Value [Member] Withholding taxes paid on share-based compensation Assets and Liabilities Measured on Nonrecurring Basis [Abstract] Fair Value Measurements, Nonrecurring Value Measurement [Abstract] Total Fair Value, Net Asset (Liability) Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract] Fair Value Measurement Inputs and Valuation Techniques [Abstract] Asset Class [Domain] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Domain] Recurring [Member] FAIR VALUE [Abstract] Fair Value Measurement Inputs and Valuation Techniques [Table] Fair Value Measurement Inputs and Valuation Techniques [Line Items] Asset Class [Axis] Contract amount of commitments and contingencies Fair Value Disclosure, Off-balance Sheet Risks, Face Amount, Liability Fair Value, Measurement Frequency [Domain] Measurement Frequency [Axis] Non-recurring [Member] Measurement Basis [Axis] Fair Value Measurements, Recurring and Nonrecurring [Table] Fair value of assets acquired, excluding acquired cash and intangibles FAIR VALUE Fair Value Disclosures [Text Block] Significant Unobservable Inputs, Fair Value Valuation Techniques Fair Value, Assets and Liabilities Measured on Nonrecurring Basis, Valuation Techniques [Table Text Block] Assets and Liabilities Measured on a Non-Recurring Basis Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Carrying Amounts and Estimated Fair Values of Other Financial Instruments Fair Value, by Balance Sheet Grouping [Table] Fair Value Measurement [Domain] Level 1 [Member] Level 2 [Member] Level 3 [Member] Subordinated debt held by unconsolidated subsidiary trusts Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items] Fair Value Hedging [Member] Other Federal Home Loan Bank borrowings Loans Financing Receivable [Policy Text Block] Non-PCD Loans [Member] Financial Asset Acquired and No Credit Deterioration [Member] Financial Instrument Performance Status [Domain] Financial Instrument Performance Status [Axis] Purchased Credit Deteriorated [Member] Purchased Credit Deteriorated ("PCD") [Member] PCD Loans [Member] Financial Instrument [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] Financing Receivables, Period Past Due [Axis] Sales 2017 Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year Financing Receivables, Period Past Due [Domain] Total past due Financing Receivable, Past Due 2019 Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year 2016 2018 Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year Purchases Loans acquired TDRs, amount Interest income on nonaccrual loans Loans modified in TDR during the year, number Loans modified in TDR during the year, amount Aged analysis of the company's loans [Abstract] Revolving loans amortized cost basis Troubled Debt Restructurings (TDRs) [Abstract] Financing Receivable, Troubled Debt Restructuring [Abstract] Financing Receivable Portfolio Segment [Axis] 2020 Financing Receivable, Year One, Originated, Current Fiscal Year Class of Financing Receivable [Axis] Class of Financing Receivable, Type [Axis] Current Prior Recoveries Financing Receivable, Allowance for Credit Loss, Recovery Nonaccrual Financing Receivable Portfolio Segment [Domain] Class of Financing Receivable [Domain] Class of Financing Receivable, Type [Domain] Financing Receivable, Recorded Investment [Line Items] 90 + days past due and still accruing Total allowance for credit losses [Roll Forward] Financing Receivable, Allowance for Credit Loss [Roll Forward] Addition to allowance for credit losses Carrying Amounts of Loans Purchased and Sold [Abstract] Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract] Financing Receivable, Allowance for Credit Losses [Line Items] Loans by Credit Quality Indicator Financing Receivable Credit Quality Indicators [Table Text Block] Charge-offs Financing Receivable, Allowance for Credit Loss, Writeoff Ending balance Beginning balance Financing Receivable, Allowance for Credit Loss Financing Receivable, Modifications [Line Items] Finite-Lived Intangible Assets by Major Class [Axis] Jul - Dec 2020 2021 Finite-Lived Intangible Asset, Expected Amortization, Year One Finite-Lived Intangible Assets, Major Class Name [Domain] Net carrying amount Finite-Lived Intangible Assets, Net Gross carrying amount 2022 Finite-Lived Intangible Asset, Expected Amortization, Year Two 2023 Finite-Lived Intangible Asset, Expected Amortization, Year Three Accumulated amortization Finite-Lived Intangible Assets, Accumulated Amortization 2024 Finite-Lived Intangible Asset, Expected Amortization, Year Four Identifiable Intangible Assets [Abstract] Intangible Assets [Abstract] Finite-Lived Intangible Assets [Line Items] Estimated aggregate amortization expense for each of five succeeding fiscal years [Abstract] Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract] Estimated useful life Forward Sales Commitments [Member] Forward Contracts [Member] Goodwill, net, activity GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS Goodwill and Intangible Assets Disclosure [Text Block] Goodwill Goodwill, net, beginning of period Goodwill, net, end of period Goodwill, net Goodwill Increase (decrease) in goodwill as a result of fair value adjustment Components of goodwill [Abstract] Goodwill [Roll Forward] GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS [Abstract] Cumulative amount of fair value hedging adjustment included in the carrying amount of the hedged assets Carrying amount of the hedged assets Hedging Relationship [Domain] Hedging Designation [Axis] Hedging Designation [Domain] Eliminations [Member] Intersegment Eliminations [Member] Average carrying balance of individually evaluated impaired loans Impaired Financing Receivable, Average Recorded Investment Non-Business Impaired Loans Impaired Financing Receivables [Table Text Block] Specifically allocated allowance Valuation allowance Interest income on individually evaluated impaired loans Impaired Financing Receivable, Interest Income, Accrual Method Loans with allowance allocation Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance Loans without allowance allocation Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance Impaired Impaired Financing Receivable, Recorded Investment Carrying balance Impaired Financing Receivable, 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 Decrease in other assets Increased in accrued interest and fees receivable Increase in deferred compensation arrangements Net increase in deposits Increase in deposits Adjustment to loans Decrease in loans Change in other assets and other liabilities Increase (Decrease) in Other Operating Assets and Liabilities, Net Increase in other liabilities Increase (Decrease) in Other Operating Liabilities 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 Core deposit intangibles & Other intangibles Total interest income Interest and Dividend Income, Operating Interest and fees on loans Interest income: Total interest expense Interest Expense Interest expense: Interest on subordinated debt held by unconsolidated subsidiary trusts Interest Expense, Junior Subordinated Debentures Interest on deposits Interest on subordinated notes payable Interest and dividends on nontaxable investments Net interest income after provision for credit losses Interest Income (Expense), after Provision for Loan Loss Net interest income Net interest income Interest and dividends on taxable investments Interest Rate Swaps [Member] Interest Rate Swap Agreements [Member] Cash paid for interest Accrued interest and fees receivable Interest Receivable Interest-bearing deposits Internal Credit Assessment [Axis] Internal Credit Assessment [Axis] Internal Credit Assessment [Domain] Amortized Cost and Estimated Fair Value of Investment Securities Federal Home Loan Bank Common Stock [Member] Investment in Federal Home Loan Bank Stock [Member] INVESTMENT SECURITIES [Abstract] Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity Investments Classified by Contractual Maturity Date [Table Text Block] INVESTMENT SECURITIES Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] Subordinated debt held by unconsolidated subsidiary trusts Junior Subordinated Debenture Owed to Unconsolidated Subsidiary Trust Terms of preferred securities for each trust [Abstract] Junior Subordinated Debenture Owed to Unconsolidated Subsidiary Trust [Abstract] Subordinated notes payable Subordinated notes payable LIBOR [Member] Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Salaries and employee benefits Fair value of liabilities assumed Liabilities: Liabilities [Abstract] Increase (decrease) in other liabilities as a result of reclassification and adjustment Total liabilities and shareholders' equity Liabilities and Equity Total liabilities Liabilities Impaired loans [Abstract] Commitments to Originate Real Estate Loans for Sale [Member] Loan Origination Commitments [Member] Business Lending Loans by Credit Quality Category [Abstract] Loans and Leases Receivable, before Fees, Gross [Abstract] Loans and Leases Receivable Disclosure [Table] Loans and Leases Receivable Disclosure [Line Items] Loans held for sale, at fair value Financing Receivable, Held-for-Sale, Not Part of Disposal Group, after Valuation Allowance Allowance for credit losses Beginning balance Ending balance Loans and Leases Receivable, Allowance Allowance for Credit Losses - Loans Loans and Leases Receivable, Allowance for Loan Losses Policy [Policy Text Block] Loans Gross loans, including deferred origination costs Total loans Loans and Leases Receivable, Gross Troubled Debt Restructuring Troubled Debt Restructuring [Policy Text Block] Net loans Loans and Leases Receivable, Net Amount Loans [Member] Loans Receivable [Member] 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 Gain on sale of investment securities, net Gain on sale of investment securities, net Marketable Securities, Realized Gain (Loss) Investment Securities Marketable Securities, Policy [Policy Text Block] us-gaap_MeasurementInputCostToSellMember Measurement Input Type [Domain] Weighted Average Constant Prepayment Rate [Member] Measurement Input Type [Axis] Weighted Average Discount Rate [Member] Measurement Input, Discount Rate [Member] Percent ownership of unconsolidated subsidiary trusts 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] COVID-19 [Member] Natural Disasters and Other Casualty Events [Member] Net cash provided by operating activities Net Cash Provided by (Used in) Operating Activities Net income Net income Investing activities: Operating activities: Net cash (used in) provided by investing activities Net Cash Provided by (Used in) Investing Activities Income available to common shareholders Net Income (Loss) Available to Common Stockholders, Basic Net cash provided by (used in) financing activities Net Cash Provided by (Used in) Financing Activities Financing activities: Adjustments for New Accounting Pronouncement [Member] New Accounting Pronouncements or Change in Accounting Principle [Table] New Accounting Pronouncements or Change in Accounting Principle [Line Items] New Accounting Pronouncements New Accounting Pronouncements [Abstract] Accounting Standards Update and Change in Accounting Principle [Abstract] Acquisitions: Noninterest-bearing deposits Total noninterest expenses Noninterest Expense Total noninterest revenues Noninterest revenues Noninterest Income Noninterest expenses: Noninterest Expense [Abstract] Noninterest revenues: Total Financing Receivable, before Allowance for Credit Loss Loans [Abstract] Loans receivable, net [Abstract] Not Designated as Hedging Instrument [Member] Occupancy and equipment Liabilities for off-balance-sheet credit exposures [Roll Forward] Ending balance Beginning balance Off-Balance Sheet, Credit Loss, Liability Provision Off-Balance Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal) Operating Segments [Member] Direct expenses 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), Reclassification Adjustment from AOCI for Sale of Securities, Tax Tax effect Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, Tax Amortization of actuarial losses included in net periodic pension cost, net Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, after Tax Reclassification adjustment for net gains included in net income, gross Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax Amortization of actuarial losses included in net periodic pension cost, gross Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax Reclassification adjustment for net gains included in net income, net Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax Amortization of prior service cost included in net periodic pension cost, net Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, after Tax Tax effect Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, Tax Amortization of prior service cost included in net periodic pension cost, gross Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, before Tax Other assets Other Assets Other Intangibles [Member] Other Intangibles [Member] Tax effect OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, Tax Net unrealized holding gains arising during period, gross OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment and Tax Other operating expenses Net unrealized holding gains arising during period, net OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax Amortized cost Other Investments and Securities, at Cost Other expenses Other Noninterest Expense Other real estate owned Other real estate owned Post-retirement Benefits [Member] Other Postretirement Benefits Plan [Member] Other Assets [Member] Unrealized gains on available-for-sale securities: Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax, Portion Attributable to Parent [Abstract] Other comprehensive income, net of tax Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent Other comprehensive 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 Other comprehensive income related to unrealized 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 Pension and other post retirement obligations: Performing [Member] Performing Financial Instruments [Member] Aged Analysis of Past Due Loans by Segment Pass [Member] Cash received (paid) for acquisitions, net of cash acquired of $55,973 and $0, respectively Payments for (Proceeds from) Businesses and Interest in Affiliates Net increase in loans Payments for (Proceeds from) Loans and Leases Purchases of treasury stock Payments for Repurchase of Common Stock Real estate tax credit investments Payments for (Proceeds from) Real Estate Partnership Investment, Net Cash dividends paid Payments of Ordinary Dividends, Common Stock Cash Payments to Acquire Businesses, Net of Cash Acquired Purchases of available-for-sale investment securities Payments to Acquire Debt Securities, Available-for-sale Purchases of premises and equipment, net Payments to Acquire Property, Plant, and Equipment Pension Benefits [Member] BENEFIT PLANS Retirement Benefits [Text Block] Investment Securities Pledged as Collateral [Abstract] Pledged Financial Instruments, Not Separately Reported, Securities, by Type of Security [Abstract] Investment securities pledged to collateralize certain deposits and borrowings Portion at Fair Value Measurement [Member] [Default] Portion at Fair Value Measurement [Member] Preferred stock, shares issued (in shares) Preferred stock, par value (in dollars per share) Preferred stock, $1.00 par value, 500,000 shares authorized, 0 shares issued Preferred stock, shares authorized (in shares) Net decrease in borrowings Proceeds from (Repayments of) Debt Issuance of common stock Proceeds from Issuance of Common Stock Proceeds from maturities and redemptions of equity and other investment securities Proceeds from maturities, calls, and paydowns of available-for-sale investment securities Proceeds from sales of available-for-sale investment securities Proceeds from sales of securities Sales of treasury stock Legal and professional fees Premises and equipment, net Property, Plant and Equipment, Net Provision Provision for credit losses Provision for credit losses Provision for credit losses Receivable [Domain] LOANS [Abstract] Residential Portfolio Segment [Member] Consumer Mortgage [Member] Residential Mortgage [Member] Retained earnings Retained earnings Retained Earnings [Member] Retirement Plan Type [Axis] Retirement Plan Tax Status [Axis] Retirement Plan Funding Status [Domain] Retirement Plan Tax Status [Domain] Retirement Plan Funding Status [Axis] Retirement Plan Type [Domain] Unearned revenue Revenue, Remaining Performance Obligation, Amount 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] Estimated Fair Value of Assets Acquired and Liabilities Assumed 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] Schedule of Financing Receivables Past Due [Table] Schedule of Financing Receivable, Allowance for Credit Losses [Table] Amounts Related to Cumulative Basis Adjustments for Fair Value Hedges Schedule of Financing Receivable, Recorded Investment, Credit Quality Indicator [Table] Schedule of Finite-Lived Intangible Assets [Table] Fair Value Measured on a Recurring Basis Reconciliation of Basic to Diluted Earnings per Share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Schedule of Financing Receivable, Troubled Debt Restructurings [Table] Gross Carrying Amount and Accumulated Amortization of Identifiable Intangible Assets Schedule of Finite-Lived Intangible Assets [Table Text Block] Schedule of Business Acquisitions, by Acquisition [Table] Schedule of Available-for-sale Securities [Table] Debt Securities, Available-for-sale [Line Items] Schedule of Defined Benefit Plans Disclosures [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 Fair Value of Derivative Instruments Loans Receivable, Net Schedule of Accounts, Notes, Loans and Financing Receivable [Table Text Block] Securities sold under agreement to repurchase, short-term Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure Securities Sold under Agreements to Repurchase [Member] Securities sold under agreement to repurchase, short-term Securities Sold under Agreements to Repurchase Information about reportable segments [Abstract] Segment Reporting Information [Line Items] Segments [Domain] SEGMENT INFORMATION [Abstract] SEGMENT INFORMATION Segment Reporting Disclosure [Text Block] Preferred Debt [Member] Senior Notes [Member] Mortgage servicing rights, measurement input Mortgage servicing rights Stock-based compensation Short-term Debt, Type [Domain] Short-term Debt, Type [Axis] 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] Statement [Table] Segments [Axis] Statement [Line Items] CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) [Abstract] Equity Components [Axis] CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (Unaudited) [Abstract] Treasury stock issued to benefit plans, net (in shares) Stock Issued During Period, Shares, Employee Benefit Plan Number of common shares authorized to be repurchased (in shares) Stock issued for acquisition (in shares) Treasury stock issued to benefit plans, net Common stock issued Stock Issued Stock issued for acquisition Total shareholders' equity Balance Balance Stockholders' Equity Attributable to Parent Shareholders' equity: Shareholders' equity: Stockholders' Equity Attributable to Parent [Abstract] Cumulative Basis Adjustments for Fair Value Hedges [Abstract] 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] Financial Instruments [Domain] Treasury stock, shares at cost (in shares) Number of common shares repurchased (in shares) Treasury stock, at cost (159,119 shares, including 157,897 shares held by deferred compensation arrangements at June 30, 2020 and 180,803 shares including 179,548 shares held by deferred compensation arrangements at December 31, 2019, respectively) Treasury Stock, Value Treasury Stock [Member] Troubled Debt Restructurings on Financing Receivables Accounting Standards Update [Axis] Accounting Standards Update [Domain] Unfunded Plan [Member] Unallocated Financing Receivables [Member] Unbilled fee revenue Unbilled Contracts Receivable Unusual or Infrequent Item, or Both [Domain] Unusual or Infrequent Item, or Both [Axis] 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 Valuation Approach and Technique [Domain] Valuation Approach and Technique [Axis] Variable Rate [Domain] Variable Rate [Axis] 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] Credit Rating, Standard & Poor's [Axis] Cumulative Effect, Period of Adoption [Domain] Cumulative Effect, Period of Adoption [Axis] Cumulative Effect Adjustment [Member] Cumulative Effect, Period of Adoption, Adjustment [Member] External Credit Rating, Standard & Poor's [Domain] Maximum [Member] Minimum [Member] Ownership [Domain] Ownership [Axis] Statistical Measurement [Axis] Statistical Measurement [Domain] Scenario [Domain] Forecast [Member] Geographical [Domain] Geographical [Axis] Scenario [Axis] Weighted Average [Member] Cover [Abstract] Entity Address, Address Line One Amendment Flag City Area Code Entity Address, City or Town Current Fiscal Year End Date Document Period End Date Entity Incorporation, State or Country Code Local Phone Number Entity Address, Postal Zip Code Entity Address, State or Province Title of 12(b) Security Trading Symbol Security Exchange Name Entity Current Reporting Status Entity Filer Category Entity Registrant Name Entity Central Index Key Entity Tax Identification Number Entity Common Stock, Shares Outstanding Document Fiscal Year Focus Document Fiscal Period Focus Document Type Document Quarterly Report Document Transition Report Entity Interactive Data Current Entity File Number Entity Shell Company Entity Emerging Growth Company Entity Small Business New York [Member] Measurement inputs used to determine the present value of estimated net servicing income, including estimates of the cost of servicing loans, appropriate discount rate and prepayment speeds. Measurement Input, Embedded Servicing Value [Member] 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 Non-credit (discount) premium at acquisition of acquired loans not accounted for as debt securities with evidence of deterioration of credit quality. Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, Non-Credit (Discount) Premium Non-credit discount at acquisition Par value of loans at acquisition of acquired loans not accounted for as debt securities with evidence of deterioration of credit quality. Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Par Value Par value of loans at acquisition 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] 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 Allowance for credit losses at acquisition at acquisition of acquired loans not accounted for as debt securities with evidence of deterioration of credit quality. Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, Allowance for Credit Losses Allowance for credit losses at acquisition Principal amount of mortgage loans held for sale. Mortgage loans held for sale, principal value Mortgage loans held for sale, at principal value 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] MBVT Statutory Trust I [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. Community Statutory Trust 4 [Member] Community Capital Trust IV [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. Steuben Statutory Trust II [Member] Steuben Statutory Trust II [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. Kinderhook Capital Trust [Member] Kinderhook Capital Trust [Member] Unconsolidated subsidiary trust created for the issuance of debt. Business Trusts [Member] Business Trust [Member] 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 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 Refers to subordinated notes maturing in February 2028. Subordinated Notes Maturing in February 2028 [Member] Refers to subordinated notes maturing in November 2025. Subordinated Notes Maturing in November 2025 [Member] The number of new counties where the bank has extended its footprints. Number of New Counties Where Bank Has Extended Footprints Number of new counties where the bank has extended footprints Portion of the State of New York lying in the north of the New York metropolitan area. Upstate New York [Member] Upstate New York [Member] Western part of the State of New York. Western New York State [Member] Western New York State [Member] Refers to the number of branch locations added upon merger. Number of Branch Locations Added upon Merger Number of branch locations The number of counties where the bank operates or has branches. Number of Counties Where Bank Has Facilities Number of counties where the bank has facilities Financial services provider business headquartered in Liverpool, New York. Wealth Resources Network, Inc [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 A parent company of Steuben Trust Company, a New York State chartered community bank headquartered in Hornell, New York. Steuben Trust Corporation [Member] Steuben [Member] Parent company of The National Union Bank of Kinderhook headquartered in Kinderhook, New York. Kinderhook Bank Corp [Member] Amount of subordinated notes payable 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, Noncurrent Liabilities, Subordinated Notes Payable Subordinated notes payable 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 deposits acquired in an acquisition. Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deposits Acquired Deposits Amount of loans acquired in an acquisition. Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Loans Acquired Loans, net of allowance for credit losses on PCD loans Represents the other businesses acquired by the entity during the period. Other Acquisitions [Member] Other [Member] Amount of Federal Home Loan Bank borrowings 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, Noncurrent Liabilities, Other Federal Home Loan Bank Advances Other Federal Home Loan Bank borrowings Tabular disclosure regarding loans acquired during the period. Summary of loans acquired [Table Text Block] Loans Acquired The entire disclosure for mandatorily redeemable preferred securities due to sponsorship of business trusts. MANDATORILY REDEEMABLE PREFERRED SECURITIES [Text Block] MANDATORILY REDEEMABLE PREFERRED SECURITIES MANDATORILY REDEEMABLE PREFERRED SECURITIES [Abstract] Measurement input using estimated cost of disposal/market adjustment. Measurement Input Estimated Cost of Disposal or Market Adjustment [Member] Estimated Cost of Disposal/Market Adjustment [Member] Valuation technique calculating fair value of collateral. Valuation Technique Fair Value of Collateral [Member] Fair Value of Collateral [Member] Value of input used to measure impaired loans. Impaired Loans Measurement Input Impaired loans, measurement input Measurement input using adequate compensation. Measurement Input, Adequate Compensation [Member] Adequate Compensation [Member] Value of input used to measure servicing asset. Servicing Asset, Measurement Input2 Mortgage servicing rights, measurement input 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] This is the amount of impaired loans carried at fair value. Impaired Loans Fair Value Disclosure Impaired loans Fair value portion of borrowing from Overnight Federal Home Loan Bank with overnight maturities. Overnight Federal Home Loan Bank Borrowings, Fair Value Disclosure Overnight Federal Home Loan Bank borrowings Fair value portion of borrowing from Federal Home Loan Bank classified as other. Other Federal Home Loan Bank Borrowings, Fair Value Disclosure Other Federal Home Loan Bank borrowings Fair value portion of subordinated notes payable. Subordinated Notes Payable, Fair Value Disclosure Subordinated notes payable This item represents other liabilities, obligations incurred and payable, pertaining to costs that are statutory in nature, are incurred on contractual obligations, or accumulate over time and for which invoices have not yet been received or will not be rendered. Accrued Interest and Other Liabilities [Member] Accrued Interest and Other Liabilities [Member] Stock Repurchase Program [Abstract] Stock Repurchase Program [Abstract] 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] Amount of expense related loan transactions, lease transactions, credit loss from transactions other than loan and lease transactions, and other loss, excluding acquisition-related provision for credit losses based on assessment of uncollectability from the counterparty to reduce the account to their net realizable value. Provision for Loan, Lease, and Other Losses Excluding Acquisition-Related Provision for Credit Losses Provision Increase (Decrease) in allowance for credit losses to total loans ratio over the same quarter in prior year. Increase (Decrease) in Allowance for Credit Losses to Total Loans Ratio Over the Same Quarter in Prior Year Increase in allowance for credit losses to total loans ratio over the same quarter in prior year Amount of acquisition of off-balance sheet credit exposure which had previously been fully or partially written-off. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging. Off-Balance Sheet, Credit Loss, Liability, Acquisition Steuben acquisition Percentage of peak unemployment due to decrease in economic activity. Percentage of Peak Unemployment Percentage of peak unemployment 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] Purchased Credit Impaired [Member] Ratio of allowance for credit losses to total loans. Allowance for Credit Losses to Total Loans Ratio Allowance for credit losses to total loans ratio Amount of recovery of off-balance sheet credit exposure which had previously been fully or partially written-off. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging. Off-Balance Sheet, Credit Loss, Liability, Recovery Recoveries Amount of increase in allowance for credit loss on financing receivable from acquisition. Financing Receivable, Allowance for Credit Loss, Acquisition Steuben acquisition Amount that has been written off for expected credit loss for off-balance sheet credit exposure. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging. Off-Balance Sheet, Credit Loss, Liability, Charge-offs Charge-offs Increase (Decrease) in allowance for credit losses to total loans ratio over prior year end. Increase (Decrease) in Allowance for Credit Losses to Total Loans Ratio Over Prior Year End Increase in allowance for credit losses to total loans ratio over the prior year end Aggregate value of stock issued during the period as a result of employee stock ownership plan (ESOP), net of impact of the net settlement of tax withholding obligations. Stock Issued During Period, Value, Employee Stock Ownership Plan, Net of Impact of Net Settlement of Tax Withholding Obligations Common stock activity under employee stock ownership plan 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) Number of shares of capital stock issued (purchased by employees) in connection with an employee stock ownership plan (ESOP), net of impact of the net settlement of tax withholding obligations. Stock Issued During Period, Shares, Employee Stock Ownership Plan, Net of Impact of Net Settlement of Tax Withholding Obligations Common stock activity under employee stock ownership plan (in shares) 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 Outstanding stock that is subordinate to all other stock of the issuer. Common Stock Outstanding [Member] Common Stock Shares Outstanding [Member] A- or better credit rating as defined by the external credit rating agency, Standard & Poor's. Standard & Poor's, A- or Better Rating [Member] A- or Better [Member] Amount of investment in equity and other security measured at fair value with change in fair value recognized in other comprehensive income, in continuous unrealized loss position. Equity and Other Securities, Continuous Unrealized Loss Position Total Number of investments in equity and other securities measured at fair value with change in fair value recognized in other comprehensive income, in continuous unrealized loss position for 12 months or longer. Equity and Other Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Positions 12 months or longer Amount of investment in equity and other security measured at fair value with change in fair value recognized in other comprehensive income, in continuous unrealized loss position for less than 12 months. Equity and Other Securities, Continuous Unrealized Loss Position, Less than 12 Months Less than 12 months Number of investments in equity and other securities measured at fair value with change in fair value recognized in other comprehensive income, in continuous unrealized loss position for less than 12 months. Equity and Other Securities, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions Less than 12 months Amount of investment in equity and other security measured at fair value with change in fair value recognized in other comprehensive income, in continuous unrealized loss position for more than 12 months. Equity and Other Securities, Continuous Unrealized Loss Position, 12 Months or Longer 12 months or longer Equity and Other Securities, Co