Document and Entity Information Document Information [Table] Legal Entity [Axis] Entity [Domain] Represents the Retirement Plan Number 002. E B P 002 [Member] Community Financial System, Inc. 401(k) Employee Stock Ownership Plan Document Information [Line Items] Document and Entity Information Document Type Entity Registrant Name Entity Central Index Key Amendment Flag Statements of Net Assets Available for Benefits EBP, Statement of Net Asset Available for Benefit [Table] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Domain] Fair Value, Inputs, Level 1, Level 2, and Level 3 [Member] Fair value Fair Value Measured at Net Asset Value Per Share [Member] Net asset value NAV Investment Type [Axis] Investments [Domain] Mutual Fund [Member] Mutual funds EBP, Employer, Common Stock [Member] Common stock of plan sponsor Community Financial System, Inc. common stock EBP, Self-Directed Brokerage Account [Member] Self-Directed Brokerage Self-directed brokerage EBP, Statement of Net Asset Available for Benefit [Line Items] Statements of Net Assets Available for Benefits EBP, Asset [Abstract] Assets EBP, Investment, Fair Value [Abstract] Investments, at fair value/ at net asset value: EBP, Investment, Excluding Plan Interest in Master Trust, Fair Value Total investments at fair value/ at NAV Fair value of shares EBP, Investment, Excluding Plan Interest in Master Trust, Fair Value, Type [Extensible Enumeration] EBP, Investment, Excluding Plan Interest in Master Trust, Fair Value, Type [Extensible Enumeration] EBP, Investment, Contract Value [Abstract] Investments, at contract value: EBP, Investment, Excluding Plan Interest in Master Trust, Contract Value Stable value fund EBP, Investment, Excluding Plan Interest in Master Trust, Fair and Contract Value Total investments EBP, Receivable [Abstract] Receivables: EBP, Employer Contribution, Receivable Contributions receivable - employer EBP, Note Receivable from Participant Notes receivable from participants, net EBP, Receivable Total receivables EBP, Net Asset Available for Benefit End of year Beginning of year Net assets available for benefits per financial statements Net assets available for benefits Statements of Changes in Net Assets Available for Benefits EBP, Statement of Change in Net Asset Available for Benefit [Table] EBP, Statement of Change in Net Asset Available for Benefit [Line Items] Statements of Changes in Net Assets Available for Benefits EBP, Change in Net Asset Available for Benefit, Increase [Abstract] Additions EBP, Change in Net Asset Available for Benefit, Increase from Participant Contribution Employee contributions EBP, Change in Net Asset Available for Benefit, Increase from Employer Contribution, Cash and Noncash Employer contributions EBP, Change in Net Asset Available for Benefit, Increase from Interest and Dividend Income on Investment Interest and dividend income EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Gain (Loss) on Investment Net appreciation in the fair value of investments EBP, Change in Net Asset Available for Benefit, Increase Total additions EBP, Change in Net Asset Available for Benefit, Decrease [Abstract] Deductions EBP, Change in Net Asset Available for Benefit, Decrease from Payment to Participant Benefit payments EBP, Change in Net Asset Available for Benefit, Decrease from Administrative Expense Administrative fees Amount recorded as a reduction to net assets available for benefits for deemed distributions related to ESOP shares under applicable tax rules. Employee Benefit Plan, Change in Net Asset Available for Benefit, Reserve for Deemed Distribution Reserve for deemed distributions EBP, Change in Net Asset Available for Benefit, Decrease Total deductions EBP, Change in Net Asset Available for Benefit, Increase (Decrease) Net increase in net assets available for benefits per financial statements Net increase in net assets available for benefits EBP, Change in Net Asset Available for Benefit, Increase (Decrease) [Abstract] Net assets available for benefits Description of the Plan EBP, Description of Plan [Table] EBP, Description of Plan [Line Items] Description of the Plan Investments EBP, Description of Plan [Text Block] Description of the Plan Significant Accounting Policies EBP, Accounting Policy [Table] EBP, Accounting Policy [Line Items] Significant Accounting Policies EBP, Summary of Accounting Policy [Text Block] Significant Accounting Policies No definition available. Investments The entire disclosure for investments held by employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employee Benefit Plan, Investments [Text Block] Investments Fair Value Measurements EBP, Investment, Fair Value and NAV [Table] EBP, Investment, Fair Value and NAV [Line Items] Fair Value Measurements EBP, Investment, Fair Value and NAV [Text Block] Fair Value Measurements Transactions with Parties-in-Interest EBP, Related Party and Party-in-Interest Transactions [Table] EBP, Related Party and Party-in-Interest Transactions [Line Items] Transactions with Parties-in-Interest EBP, Related Party and Party-in-Interest Transactions [Text Block] Transactions with Parties-in-Interest Income Tax Status EBP, Tax Status [Table] EBP, Tax Status [Line Items] Income Tax Status EBP, Tax Status [Text Block] Income Tax Status Reconciliation of Financial Statements to Form 5500 EBP, Reconciliation of Financial Statement to Form 5500 [Table] EBP, Reconciliation of Financial Statement to Form 5500 [Line Items] Reconciliation of Financial Statements to Form 5500 EBP, Reconciliation of Financial Statement to Form 5500 [Text Block] Reconciliation of Financial Statements to Form 5500 Economic Dependency and Concentration of Credit Risk EBP, Risk and Uncertainty [Table] EBP, Risk and Uncertainty [Line Items] Economic Dependency and Concentration of Credit Risk EBP, Risk and Uncertainty [Text Block] Economic Dependency and Concentration of Credit Risk Schedule of Assets (Held at End of Year) EBP, Schedule of Asset Held for Investment [Table] EBP, Schedule of Asset Held for Investment [Line Items] Schedule of Assets (Held at End of Year) EBP, Schedule of Asset Held for Investment [Text Block] Schedule of Assets (Held at End of Year) EBP, Basis of Accounting [Policy Text Block] Basis of Accounting EBP, Investment [Policy Text Block] Valuation of Investments EBP, Payment to Participant [Policy Text Block] Payment of Benefits EBP, Expense [Policy Text Block] Administrative Expenses Disclosure of accounting policy for inactive accounts of employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employee Benefit Plan, Inactive Accounts [Policy Text Block] Inactive Accounts EBP, Use of Estimate [Policy Text Block] Use of Estimates EBP, Risk and Uncertainty [Policy Text Block] Risks and Uncertainties Disclosure of accounting policy for plan provisions of employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employee Benefit Plan, Plan Provisions [Policy Text Block] Plan Provisions Disclosure of accounting policy for valuation allowance of employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employee Benefit Plan, Valuation Allowance [Policy Text Block] Valuation Allowance EBP, Subsequent Event [Policy Text Block] Subsequent Events EBP, Investment, Fair Value and NAV [Table Text Block] Schedule of financial assets that were accounted for at fair value on a recurring basis EBP, Reconciliation of Financial Statement to Form 5500 [Table Text Block] Schedule of reconciliation as per the financial statements to Form 5500 Scenario [Axis] Scenario [Domain] Represents the one-time initial automatic increase of participant contribution percentage after two years of service under the plan's automatic escalation provisions. Employee Benefit Plan, Automatic Increase In Participant Contribution, Tranche One [Member] After 2 years Represents the annual percentage increase in automatically enrolled participant contributions, applied after the initial period, until the participant elects to change the contribution rate. Employee Benefit Plan, Automatic Increase In Participant Contribution, Tranche Two [Member] Thereafter Statistical Measurement [Axis] Statistical Measurement [Axis] Statistical Measurement [Domain] Statistical Measurement [Domain] Maximum [Member] Maximum Represents the minimum age criteria for plan eligibility under (employee benefit plan (EBP)). Employee Benefit Plan, Minimum Age For Plan Eligibility Minimum age for Plan eligibility Represents the minimum service period for plan eligibility under (employee benefit plan (EBP)). Employee Benefit Plan, Minimum Service Period For Plan Eligibility Minimum service period for Plan eligibility EBP, Participant Contribution, Automatic, Deferral Rate Elective contributions on automatic enrollment Percentage of increase in eligible compensation contributed for participant automatically enrolled until changed by employee in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employee Benefit Plan, Participant Contribution, Automatic, Increase in Deferral Rate Increase in deferral percentage Represents the initial portion (first 3%) of participant contributions used to determine eligibility for employer matching contributions under the plan's matching formula. Employee Benefit Plan, Contribution By Employee, Tranche One [Member] First 3% of employee contribution Represents the subsequent portion of participant contributions (i.e., contributions exceeding the initial tranche, typically the next 3%) used to determine eligibility for employer matching contributions at a reduced matching rate under the plan's matching formula. Employee Benefit Plan, Contribution By Employee, Tranche Two [Member] Next 3% of employee contribution EBP, Participant Contribution, Pretax, Maximum Annual Compensation to Total Compensation, Percentage Maximum annual compensation percentage EBP, Participant Contribution, Pretax, Maximum Annual Compensation, Amount Maximum annual compensation EBP, Employer Contribution, Participant Compensation Matched, Percentage Eligible compensation percentage EBP, Employer Contribution, Matching Percentage Employer match percentage Amount of employer matching contribution on eligible participant contribution to employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employee Benefit Plan, Employer Contribution Matching Employer matching amount Amount of additional employer contributions to employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employee Benefit Plan, Additional Employer Contributions Additional employer contribution for WRAP Amount of qualified non-elective contributions paid by employer in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employee Benefit Plan, Qualified Non Elective Contribution Amount of qualified non-elective contributions EBP, Forfeited Nonvested Account, Reallocated to Participant Forfeited accounts allocated to eligible participants Represents the vesting percentage pursuant to employer matching contribution on eligible participant contribution to employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employee Benefit Plan, Employer Contribution Matching, Vesting Percentage Employer matching contribution, vesting percentage Represents the vesting period pursuant to employer matching contribution on eligible participant contribution to employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employee Benefit Plan, Employer Contribution Matching, Vesting Period Employer matching contribution, vesting period Represents the vesting percentage pursuant to additional employer contributions to employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employee Benefit Plan, Additional Employer Contribution, Vesting Percentage Additional employer contribution, vesting percentage Represents the vesting period pursuant to additional employer contributions to employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employee Benefit Plan, Additional Employer Contribution, Vesting Period Additional employer contribution, vesting period Minimum [Member] Minimum Receivable Type [Axis] Receivable [Domain] Represents information pertaining to residential loans. Residential Loans [Member] Residential loans Represents information pertaining to reserve plus loans. Reserve Plus Loans [Member] Reserve Plus loans EBP, Note Receivable from Participant, Account, Minimum Borrowing, Amount Minimum borrowing amount EBP, Note Receivable from Participant, Account, Maximum Borrowing, Amount Maximum borrowing amount EBP, Note Receivable from Participant, Account, Maximum Borrowing, to Total Account, Percentage Maximum borrowing percentage on vested account balance Represents the percentage points added to the prime rate to compute the interest rate on participant loans. Employee Benefit Plan, Note Receivable From Participant, Basis Spread on Prime Rate Basis spread on prime rate EBP, Asset Held for Investment, Participant Loan, Interest Rate Participant loans, interest rate Represents the participants normal retirement age. Employee Benefit Plan, Participant Normal Retirement Age Participant normal retirement age Represents the years of service completed by participant. Employee Benefit Plan, Participant Years Of Service Completed Required years of service to be completed by participant for retirement Represents the maximum vested balance for single lump sum payment in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employee Benefit Plan, Maximum Vested Balance For Single Lump Sum Payment Maximum account balance Represents the minimum vested balance for single lump sum payment in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employee Benefit Plan, Minimum Vested Balance For Single Lump Sum Payment Minimum account balance Represents the maximum age of participant up to which the participant may elect to defer payments in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employer Benefit Plan, Maximum Age Of Participant For Payment Deferral Maximum age of participant for payment deferral Represents percentage of employee contribution available for withdrawals as hardship withdrawals. Employee Benefit Plan, Hardship Withdrawal Percentage On Employee Contributions Percentage of employee contributions as hardship withdrawals Represents the percentage of vesting of participant accounts in the event of termination. Employee Benefit Plan, Vesting Percentage in Event of Termination Vesting percentage on termination Fixed Income Investments [Member] Stable Value Fund Percentage of total return on investment held by employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employee Benefit Plan, Investment, Total Return, Percentage Percentage of total return Percentage of crediting interest rate on investment held by employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employee Benefit Plan, Investment, Crediting Interest Percentage Percentage of crediting interest Processing period for participant-directed withdrawals in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employee Benefit Plan, Participant Directed Withdrawals, Processing Period Processing period EBP, Fair Value, Investment, NAV, Unfunded Commitment Unfunded commitments EBP, Related Party and Party-in-Interest [Axis] EBP, Related Party and Party-in-Interest [Domain] EBP, Trustee [Member] Trustee The amount of paid for administrative services by the entity. Employer Benefit Plan, Administrative Fees Paid Amount paid for record keeping, trustee, and other services Amount of account balances of individuals who have withdrawn from participation in the employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employees Benefit Plan, Account Balances Of Inactive Accounts Account balances of individuals withdrawn from participation EBP, Note Receivable from Participant, Allowance for Credit Loss Valuation allowance Fair Value, Inputs, Level 1 [Member] Level 1 Fair Value, Inputs, Level 2 [Member] Level 2 Person or persons designated as sponsor of employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employee Benefit Plan, Sponsor [Member] Sponsor Person or persons designated as subsidiary of employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Employee Benefit Plan, Subsidiary [Member] Subsidiary EBP, Investment, Number of Shares Number of shares Number of shares held EBP, Investment, Excluding Plan Interest in Master Trust, Cost Cost of shares EBP, Change in Net Asset Available for Benefit, Increase from Dividend Income on Investment Dividends received EBP, Investment, Sold, Number of Shares Number of shares sold EBP, Investment, Purchased, Number of Shares Number of shares purchased EBP, Tax Determination Letter, Obtained [true false] Determination letter obtained EBP, Tax Determination Letter, Date Determination letter date EBP, Tax Qualification Status [Extensible Enumeration] Tax qualification status EBP, Form 5500 Caption, Net Assets [Abstract] Reconciliation of net assets available for benefits per the financial statements to Form 5500 Amount of allowance for notes receivable from participants to reconcile net asset available for benefit per financial statement to Form 5500 caption. Form 5500 is form provided by Department of the Treasury, Internal Revenue Service (IRS); Department of Labor (DOL), Employee Benefit Security Administration (EBSA); and Pension Benefit Guaranty Corporation (PBGC). EBP, Reconciliation to Form 5500, Net Asset Available for Benefit, Allowance For Notes Receivable From Participants Allowance for notes receivable from participants EBP, Form 5500 Caption, Net Assets Net assets available for benefits per Form 5500 EBP, Form 5500 Caption, Net Income (Loss) [Abstract] Reconciliation of the net increase in net assets available for benefits per the financial statements to Form 5500 EBP, Reconciliation to Form 5500, Change in Net Asset Available for Benefit, Current Period, Decrease from Deemed Distribution Allowance utilized for deemed distributions EBP, Reconciliation to Form 5500, Change in Net Asset Available for Benefit, Prior Period, Increase from Deemed Distribution Reserve for deemed distributions EBP, Form 5500 Caption, Net Income (Loss) Net increase in net assets available for benefits per Form 5500 Investment, Issuer Name [Axis] Investment, Issuer Name [Domain] Represents the member information pertaining to Vanguard. Vanguard [Member] Vanguard Represents the member information pertaining to JP Morgan. JP Morgan [Member] JP Morgan EBP, Risk and Uncertainty, Concentration of Investment at Fair Value to Total Investments, Percentage Percentage of investments Investment, Identifier [Axis] EBP, Investment, Issuer Name [Extensible Enumeration] Investment issuer name EBP, Investment, Current Value, Type [Extensible Enumeration] Investment type EBP, Asset Held for Investment, Investment, Excluding Plan Interest in Master Trust, Current Value Current Value EBP, Asset Held for Investment, Participant Loan, after Deemed Distribution Participant loans EBP, Asset Held for Investment Total EBP, Asset Held for Investment, Party-in-Interest [true false] Investment party in interest