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Other Comprehensive Income Loss Reclassification Adjustment From AOCI For Available For Sale Securities Fair Value Hedge Before Tax Less: reclassification adjustment for net (gain) loss for fair value hedge Entity Current Reporting Status Class of Financing Receivable [Axis] Fair Value, Inputs, Level 2 [Member] Loan Restructuring Modification Name [Axis] Cash Flow Hedging [Member] Common stock, authorized (in shares) Common Stock, Shares Authorized OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, Tax Change in net unrealized gain (loss) on available-for-sale securities, Income tax benefit (expense) Product and Service [Axis] Document Fiscal Period Focus Debt Instrument, Interest Rate, Stated Percentage Debt instrument, interest rate, stated percentage Accounting Standards Update 2023-09 [Member] ASU 2023-09 Fair Value Of Interest Rate Swaps Fair value of interest rate swaps. Fair value of interest rate swaps Revenue from Contract with Customer [Abstract] Entity Shell Company Dividends, Common Stock, Cash Cash dividends declared Amortized Cost Basis Financing Receivable Troubled Loan Modification Amortized Cost Financing receivable troubled loan modification amortized cost. Federal Reserve Bank stock Federal Reserve Bank Stock Cash and Cash Equivalent Cash and cash equivalents Significant Accounting Policies [Line Items] Significant accounting policies. Amortization of net unrealized (gains) losses on securities transferred from available-for-sale to held-to-maturity Amount, before tax, of adjustment from accumulated other comprehensive income (loss) for amortization of gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) from transfer to investment in debt security measured at amortized cost (held-to-maturity). cofs_OciDebtSecuritiesTransferToHeldtomaturityFromAvailableforsaleAdjustmentFromAociForAmortizationOfNetUnrealizedGainLossBeforeTax Revenue from Contract with Customer Benchmark [Member] Total Revenues Asset Class [Domain] Certain Loans Acquired in Transfer, Accretable Yield Accretable yield Estimated accretable interest income Statement [Table] Debt securities, held-to-maturity, continuous unrealized loss position, twelve months or longer, fair value. Debt securities, held-to-maturity, continuous unrealized loss position, twelve months or longer, fair value Fair value, more than 12 months Net Increase from Security Transaction Net increase from security transaction. Net increase from security transaction Noninterest-bearing deposits Noninterest-Bearing Deposit Liabilities, Total Noninterest-Bearing Deposit Liabilities Deposits - noninterest - bearing Payments to Acquire Businesses, Gross Cash price Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year Financing Receivable, Four year prior Share Repurchase Program, Remaining Authorized, Number of Shares Shares remaining to purchase under repurchase plan Statement [Line Items] Business Combination [Domain] Amortization of net unrealized (gains) losses on securities transferred from available-for-sale to held-to-maturity, Income tax benefit (expense) Amortization of net unrealized (gains) losses on securities transferred from available-for-sale to held-to-maturity, Income tax benefit (expense) The amount of tax from oci debt securities transfer to held to maturity from available for sale adjustment from aoci for amortization of net unrealized gain (loss) Amortization of net unrealized (gains) losses on securities transferred from available-for-sale to held-to-maturity, Income tax benefit (expense) Entity Filer Category Financial Asset, 60 to 89 Days Past Due [Member] Financing Receivable, Past Due [Table] Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Intangible assets Business Combination, Recognized Asset Acquired, Identifiable Intangible Asset, Indefinite-Lived Total liabilities and shareholders’ equity Liabilities and Equity Debt Instrument [Line Items] Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date Held to Maturity Securities maturing within amortized cost Held to Maturity Securities maturing within amortized cost Financing Receivable, Allowance for Credit Loss, Recovery Recoveries Amortization Amortization Merger fair value adjustments Merger fair value adjustments Noninterest Income Total noninterest income Debt securities, held-to-maturity, continuous unrealized loss position, less than twelve months, accumulated loss. Debt Securities, Held-to-Maturity, Continuous Unrealized Loss Position, Less than twelve Months, Accumulated Loss Less than 12 months, unrealized losses Recorded Investment Fair Value, by Balance Sheet Grouping [Table Text Block] Summary of Financial Instruments Hedging Relationship [Axis] Represents the cash dividend. Cash Dividend [Member] Financial Instruments Financial Instruments Disclosure [Text Block] Effect of employee stock purchases Compensation expense for employee stock purchases Derivative [Line Items] Cash and cash equivalents Financial Asset, 30 to 59 Days Past Due [Member] AA or Better [Member] AA Or Better [Member] AA Or better. Goodwill, Total Goodwill Goodwill, Ending Balance Goodwill, Beginning Balance Goodwill Percentage of increase decrease of loans Percentage of Increase Decrease of Loans and Leases Receivable Percentage of increase decrease of loans and leases receivable. Stock Options [Member] Concentration Risk Threshold Percentage Concentration risk threshold percentage. Concentration risk percentage Real Estate Owned, Transfer to Real Estate Owned Loans transferred to other real estate Asset-backed Securities [Member] Asset-Backed Securities [Member] Derivative Weighted Average Coupon Rate Derivative weighted average coupon rate. Derivative weighted average coupon rate Equity Securities, FV-NI, Unrealized Loss Gross Unrealized Losses Cash collateral pledged Cash collateral pledged. Cash Collateral Pledged Underlying Asset Class [Domain] Purchase of Federal Home Loan Bank stock Purchase of Federal Home Loan Bank stock Payments to Acquire Federal Home Loan Bank Stock Debt securities, held-to-maturity, continuous unrealized loss position, twelve months or longer, accumulated loss. Debt securities, held-to-maturity, continuous unrealized loss position, twelve months or longer, accumulated loss Unrealized loss, more than 12 months Shares surrendered by participants for RSU tax payments Share-Based Payment Arrangement, Decrease for Tax Withholding Obligation Shares surrendered by participants for RSU tax payments Summary of Significant Accounting Policies Pre- Modification Outstanding Nonperforming Financial Instruments [Member] Non-PCD Loans [Member] Unpaid Principal Balance with an allowance recorded Financing Receivable Originated Writeoffs Five or More Years Before Latest Fiscal Year, Total Financing Receivable Originated Writeoffs Five or More Years Before Latest Fiscal Year, Total Current year-to-date gross write-offs, Prior Financing Receivable Originated Writeoffs Five or More Years Before Latest Fiscal Year Financing receivable originated writeoffs five or more years before latest fiscal year. Disaggregation of Revenue [Table] Brokered deposits Deposits, Fair Value Disclosure Schedule of Basic Earnings Per Share and Diluted Earnings Per Share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Equity Securities [Member] Special Mention [Member] Payment to Acquire Life Insurance Policy, Investing Activities Purchase of bank-owned life insurance policies Purchase of bank-owned life insurance policies Schedule of Maturities of Securities Investments Classified by Contractual Maturity Date [Table Text Block] Basic earnings per common shares Basic (loss) earnings per common shares Basic earnings (loss) per share (Note 4) Schedule of Noninterest Income Separated by Revenue Disaggregation of Revenue [Table Text Block] Repayments of Long-Term Debt Repayments of Long-Term Debt, Total Payments on borrowings and subordinated debentures Gross unrealized gains, held to maturity Debt Securities, Held-to-Maturity, Accumulated Unrecognized Gain Payments for (Proceeds from) Loans and Leases, Total Payments for (Proceeds from) Loans and Leases Loan originations and payments, net Servicing Asset at Fair Value, Amount, Ending Balance Servicing Asset at Fair Value, Amount, Beginning Balance Mortgage loan servicing rights Number of investments in debt securities measured at fair value which have been sold. cofs_DebtSecuritiesAvailableForSaleNumberOfPositionsSold Debt securities, available-for-sale, number of positions sold Related Allowance Hedging Designation [Domain] Pay Fixed Swap Agreement. Pay Fixed Swap Agreement [Member] Bank Term Funding Program maturity of january 2025. Bank Term Funding Program Maturity Of January 2025 [Member] Bank Term Funding Program Maturity of January 2025 [Member] Revenue, Major Customer [Line Items] Federal Home Loan Bank Advances Maturing December 2026. Federal Home Loan Bank Advances Maturing December 2026 Member] Federal Home Loan Bank Advances Maturing December 2026 [Member] Reclassified From Available-for-sale Securities [Member] Represents reclassified from available for sale securities. Held to Maturity Securities maturing within More than10 Years Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years Business Combination, Recognized Liability Assumed, Other Liability, Noncurrent Other liabilities Accounting Standards Update [Axis] Notes To Financial Statements Abstract Notes To Financial Statements [Abstract] Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Balance Balance Stockholders' Equity, Policy [Policy Text Block] Stock Transactions The amount of financial assets (as defined) recognized as of the federal home loan bank and federal reserve bank stock. cofs_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedFederalHomeLoanBankAndFederalReserveBankStock Federal Home Loan Bank and Federal Reserve Bank stock Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, Tax Less: reclassification adjustment for net (gain) loss on cash flow hedge, Income tax benefit (expense) Less: reclassification adjustment for net (gain) loss on cash flow hedge,Income tax benefit (expense) Equity, Attributable to Parent [Abstract] Shareholders' Equity Interest income recognized during the period on nonaccrual loans Financing Receivable, Nonaccrual, Interest Income Schedule of Changes in Level 3 Assets Measured at Fair Value on a Recurring Basis Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Entity Small Business Interest income Insurance and Investment Commissions [Member] Insurance and investment commissions. Insurance and Investment Commissions [Member] Hedging Designation [Axis] Payments to Acquire Federal Reserve Bank Stock Purchase of Federal Reserve Bank stock External Credit Rating by Grouping [Axis] Retained earnings Retained Earnings (Accumulated Deficit), Total Retained Earnings (Accumulated Deficit) Business acquisition description Business Combination, Description of Acquiree Financing Receivable, Nonaccrual [Table Text Block] Nonaccrual Loans by Loan Category and Interest Income Recognized Payments to Acquire Equity Securities, FV-NI Purchases of equity securities Debt Securities, Trading, and Equity Securities, FV-NI [Table Text Block] Schedule of Gross Unrealized Gains (Losses) Proceeds from Issuance Initial Public Offering Cash related to common stock offering Payments of Dividends, Total Payments of Dividends Cash dividends Unrated [Member] Unrated. The amount of revenue from customer services charges and fees. Customer service charges Amendment Flag Taxable OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment and Tax Change in net unrealized gain (loss) on available-for-sale securities Product and Service [Domain] Business Combination, Acquired Receivable, Purchased without Credit Deterioration, Fair Value Fair value on acquired loans Nonaccrual Financing Receivable [Member] Nonaccrual financing receivable. Proceeds from Derivative Contracts Settlements Proceeds from derivative contracts settlements. Proceeds from derivative contracts settlements Reclassification, Type [Axis] Entity Address, Postal Zip Code Cash Provided by (Used in) Operating Activity, Including Discontinued Operation Net cash (used in) provided by operating activities Debt Securities, Percent Debt Securities Percent Debt securities percent. Substandard [Member] Average Recorded Investment with an allowance recorded Segment Reporting, CODM, Profit (Loss) Measure, How Used, Description Goodwill, Period Increase (Decrease) Goodwill, Period Increase (Decrease), Total Goodwill increasing Nonaccrual loans with no ACL Financing Receivable, Nonaccrual, No Allowance Hedging Relationship [Domain] Subordinated Debt Obligations, Fair Value Disclosure Subordinated debentures Schedule of Derivatives Instruments in Balance Sheet Location Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block] Summary of Troubled Loan Modification (TLMs) Troubled Loan Modification On Financing Receivables [Table Text Block] Troubled loan modification on financing receivables. Debt Instrument, Interest Rate, Effective Percentage Debt instrument, interest rate, effective percentage Debt instrument, interest rate, effective percentage Portion at Fair Value Measurement [Member] [Default] Portion at Fair Value Measurement [Member] Entity Address, Address Line One Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false] Summary of Amortized Cost Basis Loans Modified To Borrowers Experiencing On Financing Receivable Amortized Cost Basis Loans Modified To Borrowers Experiencing On Financing Receivable [Table Text Block] Amortized cost basis loans modified to borrowers experiencing on financing receivable. Fair Value Hierarchy and NAV [Axis] Earnings Per Share [Abstract] Consumer Portfolio Segment [Member] Consumer [Member] Residential Real Estate [Member] Residential Real Estate [Member] Derivative Asset, Total Interest rate contracts Interest rate contracts Derivative assets, Fair Value Greater than 90 days [Member] Maturity Greater than 90 Days [Member] Other Liabilities [Member] Other Liabilities Statement of Financial Position Location, Balance [Axis] Tax exempt Schedule of Assets Measured at Fair Value on a Non-recurring Basis Fair Value Measurements, Nonrecurring [Table Text Block] Collateral Dependent Loans Fair Value Disclosure Collateral Dependent Loans Collateral Dependent Loans Fair Value Disclosure Total, fair value Total, fair value Debt Securities, Available-for-Sale, Unrealized Loss Position Repurchase Agreements and Similar Transactions, Maturity Periods [Axis] Payments to Acquire Debt Securities, Available-for-Sale Purchases of securities available for sale OCI, Debt Securities, Available-for-Sale, Gain (Loss), after Adjustment and Tax Unrealized gain (loss) on available-for-sale securities, net of tax Document Type Financing Receivable, Individually Evaluated for Credit Loss Individually evaluated for impairment Financing Receivable, Individually Evaluated for Credit Loss, Total Loans evaluated for allowance for credit loss Cash paid for income taxes Income Taxes Paid, Net, Total Income Taxes Paid, Net Cash value of life insurance policies Cash Surrender Value of Life Insurance Post- Modification Outstanding Entity Central Index Key Acquisition related allowance for credit loss (PCD) Allowance For Loan And Lease Losses Acquisition Related Allowance For Credit Loss Purchased With Credit Deterioration Allowance for loan and lease losses acquisition related allowance for credit loss purchased with credit deterioration. ACL Balance Represents information related to borrowings. cofs_Borrowings Borrowings Time deposits in other financial institutions Interest-Bearing Deposits in Banks and Other Financial Institutions Represents number of branch office. cofs_NumberOfBranchOffices Number of branch offices Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Period Increase (Decrease), Including Exchange Rate Effect and Discontinued Operation Net change in cash and cash equivalents Subsequent Event Type [Domain] Federal Reserve Bank , general debt obligations, disclosures, collateral pledged. Federal Reserve Bank , General Debt Obligations, Disclosures, Collateral Pledged Federal reserve bank, general debt obligations, disclosures, collateral pledged Antidilutive Securities Excluded from Computation Antidilutive securities excluded from computation of earnings per share, amount (in shares) A rated. A Rated [Member] A Rated [Member] Represents information related to County Bank Corp ("County"). County Bank Corp ("County") [Member] Accounting Standards Update [Domain] Costs capitalized to other real estate Cost Capitalized To Other Real Estate Cost capitalized to other real estate. Liabilities [Abstract] Liabilities Increase (Decrease) in Other Operating Liabilities, Total Increase (Decrease) in Other Operating Liabilities Other liabilities Amortized cost, available for sale Debt Securities, Available-for-Sale, Amortized Cost Amortized cost, available for sale Fair value, available for sale Fair value, available for sale Securities available for sale, at fair value (Note 2) Securities available for sale Total Available for Sale Represents information related to trust preferred securities. Trust Preferred Securities [Member] Noninterest income Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Exercises in Period Stock options exercised and issued (1) Stock options exercised and issued (in shares) Doubtful [Member] Debt Disclosure [Abstract] Unrealized gain (loss) on cash flow hedge instruments, net of tax Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax Federal Home Loan Bank Advances Maturing March 2026 [Member] Federal Home Loan Bank Advances maturing march 2026. Percentage of allowance for credit losses Percentage of Allowance for Credit Losses Percentage of allowance for credit losses. Borrowings from federal reserve bank Borrowings from Federal Reserve Bank Borrowings from Federal Reserve Bank. Derivative [Table] Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases Securities purchased Interest Income [Member] Proceeds from loan sales Proceeds from Sale of Loans Held-for-sale, Total Proceeds from Sale, Loan, Held-for-Sale Cash Received From Merger Cash received from merger. Cash received from merger with Fentura Financial, Inc. Cash received from merger with Fentura Financial, Inc. Supplemental Cash Flow Elements [Abstract] Supplemental disclosures of cash flow information: Agricultural and Residential Real Estate [Member] Information related to agricultural and residential real estate loans. Agricultural and Residential Real Estate [Member] Estimate of Fair Value Measurement [Member] Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] Derivative, Notional Amount Notional Amount Derivative, notional amount Financial Instrument Performance Status [Domain] Federal Home Loan Bank Advances Maturing May 2026 [Member] Federal Home Loan Bank Advances Maturing May 2026. Current Fiscal Year End Date Schedule of Long-Term Debt Instruments [Table] Gains (Losses) on Sales of Other Real Estate (Gains) on sales of other real estate owned Document Quarterly Report Business Combination, Recognized Asset Acquired, Other Asset, Noncurrent Other assets Receivables [Abstract] Unallocated Financing Receivables [Member] Fair Value, by Balance Sheet Grouping [Table] Represents information related to interchange income. Interchange Income [Member] Entity Address, State or Province Transfer from non-accretable to accretable yield Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield, Reclassifications from Nonaccretable Difference Financing Receivable Originated Writeoffs in Current Fiscal Year Financing receivable originated writeoffs in current fiscal year. Current year-to-date gross write-offs, Current year Net change in short term borrowings Proceeds from (Repayments of) Short-Term Debt, Total Proceeds from (Repayments of) Short-Term Debt Data processing Financial Instrument Performance Status [Axis] Business Combination [Table Text Block] Summary of Allocation of Purchase Price Preferred stock, outstanding (in shares) Preferred Stock, Shares Outstanding, Ending Balance Preferred Stock, Shares Outstanding, Beginning Balance Preferred Stock, Shares Outstanding Comprehensive Income (Loss), Net of Tax, Attributable to Parent Comprehensive income (loss) Gross unrealized gains, available for sale Gross unrealized gains, available for sale Federal Home Loan Bank Advances maturing january 2026. Federal Home Loan Bank Advances Maturing January 2026 [Member] Federal Home Loan Bank Advances Maturing January 2026 [Member] Interest Expense, Federal Home Loan Bank and Federal Reserve Bank Advances, Long-Term Advances from Federal Home Loan Bank Plus dilutive stock options and restricted stock units Plus dilutive stock options and restricted stock units (in shares) Business Combination, Transaction Cost, Excluding Separately Recognized Transaction Acquired securities under merger combination Internal Credit Assessment [Axis] Equity Securities, FV-NI, Unrealized Gain Gross Unrealized Gains Reclassification of unrealized gain (loss) upon transfer of securities from available-for-sale to held-to-maturity Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Investment Transferred from Available-for-Sale to Equity Method, before Tax Reclassification of unrealized gain (loss) upon transfer of securities from available-for-sale to held-to-maturity Professional fees Debt Securities, Trading, and Equity Securities, FV-NI [Table] Bank Term Funding Program Maturity of May 2024 Bank Term Funding Program Maturity Of May 2024 [Member] Bank Term Funding Program Maturity Of May 2024 [Member] Performance Shares [Member] Fair value portion of accrued interest payable. Accrued interest payable Proceeds from Sale and Maturity of Marketable Securities Sales of securities available for sale Sales of equity securities Subsequent Event [Member] Write Downs of O R E O Write Downs of OREO. Write downs of OREO Write downs of OREO Marketable Securities, Policy [Policy Text Block] Securities ACL for Securities Debt securities, held-to-maturity securities, unrealized loss position, fair value. Debt Securities, Held-to-Maturity securities, Unrealized Loss Position, Fair Value Fair value Number of loan production offices. Number of loan production offices Represents information related to other charges and fees for customer services. Other Charges and Fees for Customer Services [Member] Other charges and fees for customer services Total interest expense Interest Expense, Operating Net interest income after provision Net interest income after provision Interest Income (Expense), after Provision for Loan Loss Net interest income plus other income 1 Year - 5 Years Fair value of investment in debt security, measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI), maturing in rolling second through fifth fiscal year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date. 1 Year - 5 Years Antidilutive Securities [Axis] Allowance for credit loss Provision Expense on Securities Provision expense on securities. Stock Issued During Period Value Common Stock Offering Stock issued during period value common stock offering. Common stock offering Core Deposit Intangible Goodwill and Intangible Assets, Intangible Assets, Policy [Policy Text Block] Proceeds from Sale of Debt Securities, Available-for-Sale Proceeds from sales of securities Trust preferred Securities Fair Value Adjustment Value Amortization Period Trust preferred securities fair value adjustment value amortization period. Trust preferred securities fair value adjustment value amortization period Available-for-Sale Securities [Member] Core deposit intangible increasing Core Deposit Intangible Increasing Core deposit intangible increasing. AOCI Attributable to Parent [Member] Accumulated Other Comprehensive Income/(Loss), Net [Member] Derivative Liability, Total Interest rate contracts Derivative instruments liabilities Derivative liabilities Bank Term Funding Program Maturity of December 2024. Bank Term Funding Program Maturity Of December 2024 [Member] Bank Term Funding Program Maturity Of December 2024 [Member] Federal Home Loan Bank, Advance, Branch of FHLBank [Line Items] Federal Home Loan Bank Advances Maturing December 2026 One [Member] Federal Home Loan Bank Advances maturing december 2026 One. Business Combination, Consideration Transferred Business Combination, Consideration Transferred, Total Consideration Preferred stock; shares authorized: 100,000; shares outstanding: none Preferred Stock, Value, Issued Amount of trust preferred securities assumed at the acquisition date. cofs_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedTrustPreferredSecurities Trust preferred securities Carrying balance at acquisition date Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net Derivative, Loss on Derivative Loss on derivative Minimum [Member] Minimum [Member] Retail Loans [Member] Retail loans. Retail Loans [Member] Financial Instrument [Axis] Financing Receivable, Prior year Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year Interest Expense, Other Other The amount of bank owned life insurance income classified as operating activities. cofs_BankOwnedLifeInsuranceIncomeOperatingActivities Earnings on bank-owned life insurance Provision For Credit Losses On Unfunded Commitments Provision for credit losses on unfunded commitments. Provision for (reversal of) credit losses on unfunded commitments Federal Home Loan Bank, Advance, Branch of FHLBank, Interest Rate Interest rate Intangible amortization Intangible amortization Fair Value, Inputs, Level 3 [Member] Fair Value, Nonrecurring [Member] Customer service charges Customer Service Charges [Member] Customer Service Charges. The 3.4015 percent pay fixed swap agreement. The 3.4015 Percent Pay Fixed Swap Agreement [Member] Segment Reporting [Abstract] Business Combination, Recognized Asset Acquired, Asset Total assets Total assets The amount of tax on other comprehensive income (loss) cash flow hedge amortization of net unrealized gain (loss) reclassification. Less: amortization of net unrealized (gains) losses included in net income, Income tax benefit (expense) Amortization of net unrealized (gains) losses included in net income, Income tax benefit (expense) Less: accretion of net unrealized (gains) losses included in net income, Income tax benefit (expense) Schedule of Assets and Liabilities Measured at Fair Value Fair Value, Assets Measured on Recurring Basis [Table Text Block] Financing Receivable, Nonaccrual [Table] Consumer Loans. Consumer Loans [Member] Consumer Loans [Member] Schedule of Available for Sale Securities with Unrealize Losses Debt Securities, Available-for-Sale, Unrealized Loss Position, Fair Value [Table Text Block] Bank Term Funding Program Advances. Bank Term Funding Program Advances [Member] Bank Term Funding Program Advances [Member] Interest expense Interest Rate Contract [Member] Interest Rate Derivative Contracts Eurodollar [Member] Financing Receivable, Collectively Evaluated for Credit Loss Collectively evaluated for impairment Debt instrument, basis spread on variable rate Debt Instrument, Basis Spread on Variable Rate Common stock, par value (in dollars per share) Common Stock, No Par Value Amortized Cost Equity Securities, FV-NI, Cost Commercial Loans. Commercial Loans [Member] Concentration Risk Benchmark [Axis] Gain (Loss) on Derivative Instruments, Net, Pretax, Total Gain (Loss) on Derivative Instruments, Net, Pretax The effects of fair value and cash flow hedging: Significant Accounting Policies [Table] Significant accounting policies. Insurance and Investment Commission Income [Member] Insurance and investment commission income. Interest and Dividend Income, Operating Total interest income Prior Financing Receivable, Originated, More than Five Years before Current Fiscal Year Interest Income (Expense), Operating Net interest income Mortgage Warehouse Advances Mortgage warehouse advances. Mortgage warehouse advances (Note 3) Revolving Financing Receivable, before Allowance for Credit Loss Total loans held for investment (Note 3) Loans Ending loan balance Mortgage Warehouse Advances [Member] Mortgage warehouse advances. Business Combination, Recognized Asset Acquired, Cash and Cash Equivalent Cash and cash equivalents Amortized cost, held to maturity Debt Securities, Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss Amortized cost, held to maturity Financing Receivable, 90 Days or More Past Due, Still Accruing Loans, 90 Days Past Due and Accruing Business Loan. Business Loan [Member] Long-Term Debt, Type [Domain] Weighted average common shares outstanding Weighted average common shares outstanding (in shares) Weighted average common shares outstanding (in shares) Weighted Average Number of Shares Outstanding, Basic, Total Amount, after tax and adjustment, of unrealized gain (loss) on held-to-maturity securities recognized in other comprehensive income. cofs_OciDebtSecuritiesHeldtomaturityUnrealizedGainLossAfterAdjustmentAndTaxTotal Unrealized loss on held to maturity securities, net of tax Schedule of Unrealized Gains and Losses on Equity Securities Gain (Loss) on Securities [Table Text Block] Document Information [Line Items] More than 12 months, unrealized losses Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss Loans, net Loans, net APIC, Share-Based Payment Arrangement, Increase for Cost Recognition, Total Stock-based compensation expense APIC, Share-Based Payment Arrangement, Increase for Cost Recognition Variable Rate [Domain] Represents fair value of other real estate. Other Real Estate Other Real Estate Other Real Estate Fair Value Equity, Attributable to Parent Balance Balance Total shareholders’ equity Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax Less: reclassification adjustment for net (gain) loss included in net income, Income tax benefit (expense) Statement of Stockholders' Equity [Abstract] Gross unrealized losses, held to maturity Debt Securities, Held-to-Maturity, Accumulated Unrecognized Loss Accretion On Purchased Loans Accretion on purchased loans. Accretion on purchased loans Payments for Loans Loans remitted payments 5 Years - 10 Years Fair value of investment in debt security, measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI), maturing in rolling sixth through tenth fiscal year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date. 5 Years - 10 Years Use of Estimates Use of Estimates, Policy [Policy Text Block] Allowance for Credit Losses ("ACL") Credit Loss, Financial Instrument [Policy Text Block] Internal Credit Assessment [Domain] Business Combination, Recognized Asset Acquired, Property, Plant, and Equipment Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment, Total Premises and equipment Ending balance ACL Balance Allowance for credit loss Financing Receivable, Allowance for Credit Loss Beginning balance Allowance for loan losses (Note 3) Fair Value Measurements Fair Value Disclosures [Text Block] Noninterest Income, Other Other Payments to Acquire Held-to-Maturity Securities Purchases of securities held to maturity Held to Maturity Securities maturing within 5 Years -10 Years Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date, Rolling after 5 through 10 Years Common stock offering (in shares) Stock Issued During Period Shares Common Stock Offering Stock issued during period shares common stock offering. Debt Instrument, Name [Domain] Percentage of Total Class of Financing Receivable Percentage of total class of financing receivable. % of Total Class of Financing Receivable Derivative Instruments and Hedging Activities Disclosure [Abstract] Segment Reporting, CODM, Individual Title and Position or Group Name [Extensible Enumeration] Unlikely to be Collected Financing Receivable [Member] Loss [Member] Plan Name [Domain] Increase (Decrease) in Other Operating Assets Other assets Other assets AA or Better [Member] Related to AA or better. AA or Better [Member] Federal Home Loan Bank Advances Maturing March 2026 One [Member] Federal home loan bank advances maturing march 2026 one. Payment for Origination, Loan, Mortgage, Held-for-Sale Loans originated for sale Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Entity [Domain] Schedule Of Collateral Dependent Loans And Related Allowance For Credit Losses Allocated To Loans [Table Text Block] Schedule of collateral dependent loans and related allowance for credit losses allocated to loans. Schedule of Collateral Dependent Loans and Related Allowance For Credit Losses Allocated to Loans Total debt securities [Member] Debt Securities [Member] Securities [Member] Federal Home Loan Bank, Advance, Branch of FHLBank [Table Text Block] Schedule of Contractual Maturities of Federal Home Loan Bank Advances Unpaid Principal Balance Schedule of bank term funding program , advances. Schedule of Bank Term Funding Program , Advances [Table Text Block] Schedule of Bank Term Funding Program Advances Certain Loans Acquired in Transfer, Nonaccretable Difference Nonaccretable difference Interest Income Recognized with an allowance recorded Proceeds from Sale of Other Real Estate Proceeds from sales of other real estate owned OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), Tax Less: net unrealized (gains) losses on securities transferred from available-for-sale to held-to-maturity Net unrealized (gains) losses on securities transferred from available-for-sale to held-to-maturity, Income tax benefit (expense) Acquisition of Liabilities From Merger Acquisition of liabilities from merger. Acquisition of liabilities from merger Financing Receivable, Year One, Originated, Current Fiscal Year Term Extension Financing Receivable, Current year Financing Receivable Originated Writeoffs Three Years Before Latest Fiscal Year Financing receivable originated writeoffs three years before latest fiscal year. Current year-to-date gross write-offs, Three year prior Collateral Held [Axis] Cash and due from banks Cash and Due from Banks, Total Cash and Due from Banks Aging Analysis of Loans by Loan Category Financing Receivable, Past Due [Table Text Block] Entity Address, City or Town Pay Floating Swap Agreement. Pay Floating Swap Agreement [Member] Stock Repurchased During Period, Shares Shares repurchased (in shares) Shares repurchased (in shares) Shares repurchased Loans acquired - contractual payments, Total Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, Contractually Required Payments Receivable at Acquisition Number of Reportable Segments Number of reportable segments Number of reportable segments Security Exchange Name Debt Instrument, Variable Interest Rate, Type [Extensible Enumeration] Interest Expense, Deposits, Total Deposits Interest Expense, Deposits Noninterest expense Increase Decrease in Interest Income Increase decrease in interest income. Interest income reduced Income before income tax Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Income (Loss) before income tax expense (benefit) Common Stock and Paid in Capital [Member] Common Stock Including Additional Paid in Capital [Member] Deposits, Savings Deposits Savings deposits (Note 9) Concentration Risk Type [Domain] Purchases Without Credit Deterioration [Member] Purchases Without Credit Deterioration [Member] Purchases Without Credit Deterioration. Collectively evaluated for credit loss Financing Receivable, Allowance for Credit Loss, Collectively Evaluated Gain (Loss) on Disposition of Other Assets Net gains on sales and write downs of other assets Accretion of derivative termination (gain) loss Accretion Of Derivative Termination (gain) Loss Accretion of derivative termination (gain) loss Antidilutive Securities, Name [Domain] Change in net unrealized gain (loss) on cash flow hedge Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), before Reclassification and Tax Financial Asset, Not Past Due [Member] Cash paid for interest Interest Paid, Excluding Capitalized Interest, Operating Activity Measurement Basis [Axis] Statistical Measurement [Domain] Current [Member] Maturity Less than 30 Days [Member] Issuance of Common Stock Consideration For Merger Issuance of common stock consideration for merger. Issuance of common stock as consideration for merger Document Period End Date Fair Value Hierarchy and NAV [Domain] Average Recorded Investment with no related allowance recorded Interchange income Interchange Income Interchange income. Merger with County Bank Corp. on October 1, 2019 Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield, Additions Statement of Financial Position [Abstract] Restricted Stock Units (RSUs) [Member] Line of Credit [Member] Line of Credit [Member] Gross realized losses Debt Securities, Available-for-Sale, Realized Loss Trust fee income Represents information related to trust fee income. Trust Fee Income [Member] Trust Preferred Securities Before Merger Fair Value Adjustments Trust preferred securities before merger fair value adjustments. Trust preferred securities before merger fair value adjustments Antidilutive Security, Excluded EPS Calculation [Table] Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent [Abstract] Other comprehensive income: Federal Home Loan Bank Advances Maturing June 2026 One [Member] Federal Home Loan Bank Advances Maturing June 2026 One. Interest Income Recognized Net gains (losses) on sales of securities Debt Securities, Available-for-Sale, Realized Gain (Loss) Debt Securities, Available-for-Sale, Realized Gain (Loss), Total Commercial and Industrial and Residential Real Estate Portfolio Segment. Commercial And Industrial And Residential Real Estate Portfolio Segment [Member] Commercial And Industrial And Residential Real Estate Portfolio Segment [Member] Total advances outstanding at year-end Advance from Federal Home Loan Bank, Total Advance from Federal Home Loan Bank Not Designated as Hedging Instrument [Member] Not Designated as Hedging Instrument [Member] Financial Asset, Aging [Domain] Net change in deposits Increase (Decrease) in Deposits, Total Increase (Decrease) in Deposits Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] Liabilities Total liabilities Troubled Loan Modification [Member] Troubled Loan Modifications [Member] Troubled Loan Modifications. Troubled Loan Modifications [Member] OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), after Tax OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), after Tax Financial Instruments [Domain] Communication Communications Collateral Pledged [Member] Collateral [Member] Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Investment Transferred from Available-for-Sale to Equity Method, Tax Reclassification of unrealized gain (loss) upon transfer of securities from available-for-sale to held-to-maturity, Income tax benefit (expense) Derivative Liability, Statement of Financial Position [Extensible Enumeration] Trust preferred securities after merger fair value adjustments Trust Preferred Securities After Merger Fair Value Adjustments Trust preferred securities after merger fair value adjustments. Derivative Instruments, Gain (Loss) [Line Items] Schedule of Information Regarding Credit Exposure Financing Receivable Credit Quality Indicators [Table Text Block] Financing Receivable, Modified [Table] Recorded Investment with no related allowance recorded Variable Rate [Axis] Less than 12 months, unrealized losses Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss One SOFR Cap Agreement. One SOFR Cap Agreement [Member] Loans and Leases Receivable, Gross, Carrying Amount, Covered Core loans (Note 3) Gross Loans Stock Issued During Period, Shares, Employee Stock Ownership Plan Repurchase of shares from Fentura Financial, Inc. ESOP (in shares) Interest Income Recognized with no related allowance recorded Weighted average common shares outstanding and potentially dilutive shares Weighted average common shares outstanding and potentially dilutive shares (in shares) Weighted average common shares outstanding and potentially dilutive shares (in shares) Securities Act File Number Marketing and Advertising Expense, Total Advertising and promotional Other assets Other Assets, Total Other Assets Cover [Abstract] Debt and Equity Securities, FV-NI [Line Items] Represents total loans for business combination recognized identifiable assets acquired and liabilities. cofs_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedTotalLoans Total loans Subordinated Debt [Member] Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax Less: reclassification adjustment for net (gain) loss included in net income Business Acquisitions Adjustment To Net Income Loss After Tax Business acquisitions adjustment to net income loss after tax. Business acquisitions adjustment to net income loss after tax Financial Asset, Aging [Axis] Derivative, Gain (Loss) on Derivative, Net, Total Derivative, Gain (Loss) on Derivative, Net Total amounts of income and expense line items presented in the consolidated statements of income in which the effects of fair value or cash flow hedges are recorded U.S. Treasury Notes and Bonds [Member] US Treasury Securities [Member] Schedule of Loans for Impairment and Interest Recognized on Impaired Loans Impaired Financing Receivables [Table Text Block] Segment Reporting Segment Reporting Disclosure [Text Block] Federal Home Loan Bank Stock and Federal Reserve Bank Stock, Total Federal Home Loan Bank and Federal Reserve Bank stock December Two thousand and three offering [Member] December two thousand and three offering. December 2003 offering [Member] Mortgage-backed [Member] Collateralized Mortgage-Backed Securities [Member] Agency Mortgage Backed Securities [Member] Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity, Noncash Item [Abstract] Adjustments to reconcile net income (loss) to net cash from operating activities: Deferred Tax Liabilities Increasing Deferred tax liabilities increasing. Deferred tax liabilities increasing Merger with Fentura Financial, Inc., net of issuance costs (in shares) Stock Issued During Period, Shares, Acquisitions Revenue from Contract with Customer, Including Assessed Tax Noninterest income from contracts with customers within the scope of ASC 606 Common Stock Shares Issued Not Disclosed Common stock shares issued not disclosed. Common stock, shares issued, not disclosed Fixed rate swaps Derivative, Swaption Interest Rate Insurance and investment commissions Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Borrowing Business combination recognized identifiable assets acquired and liabilities assumed borrowing. Borrowing Business Combination, Pro Forma Information, Pro Forma Income (Loss), after Tax Net income (loss) Maturities, prepayments and calls of securities held to maturity Proceeds from Maturities, Prepayments and Calls of Held-to-Maturity Securities Recorded Investment with an allowance recorded Financing Receivable, Modifications, Number of Contracts Number of loans Derivative, Term of Contract Derivative, Term of Contract (Year) New Accounting Pronouncements, Policy [Policy Text Block] Recent Accounting Pronouncements Subordinated debentures Subordinated Debt, Ending Balance Subordinated Debt, Beginning Balance Subordinated Debt, Total Subordinated Debt Earnings Per Share [Line Items] Derivative Instruments, Gain (Loss) [Table] Number of days individually evaluation Number of Days Individually Evaluation Number of Days Individually Evaluation Total liabilities Business Combination, Recognized Liability Assumed, Liability Location and Amount of Gain (Loss) in Income on Fair Value and Cash Flow Hedging Relationships Schedule of Derivatives Instruments Statements of Financial Performance and Financial Position, Location [Table Text Block] Depreciation Depreciation, Total Depreciation Financing Receivable Originated Writeoffs Four Years Before Latest Fiscal Year Financing receivable originated writeoffs four years before latest fiscal year. Current year-to-date gross write-offs, Four year prior Reported Value Measurement [Member] Loans To Other Financial Institutions Member Loans To Other Financial Institutions [Member] Loans to other financial institutions [Member] Proceeds from borrowings Proceeds from Issuance of Long-Term Debt Proceeds from Issuance of Long-Term Debt, Total Supplies and Postage Expense, Total Supplies and postage Fentura Financial, Inc. [Member] Fentura financial, inc. Financing Receivable, Allowance for Credit Loss, Loan Purchased Provision for credit losses on acquired loans Common stock, outstanding (in shares) Common Stock, Shares, Outstanding Balance (in shares) Balance (in shares) Dividends [Domain] Schedule of Amortized Cost Basis of Loans Summary of Amortized Cost Basis of Loans [Table Text Block] Summary of Amortized Cost Basis of Loans. Document Transition Report Other comprehensive income (loss), reclassification adjustment from aoci for available-for-sale securities, fair value hedge, tax. Other Comprehensive Income Loss Reclassification Adjustment From AOCI For Available For Sale Securities Fair Value Hedge Tax Less: reclassification adjustment for net (gain) loss for fair value hedge, Income tax benefit (expense) Fair value, held to maturity Debt securities held-to-maturity fair value Fair value, held to maturity Debt Securities, Held-to-Maturity, Fair Value The amount of financial assets (as defined) recognized as of the other real estate owned. cofs_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherRealEstateOwned Other real estate owned Debt Securities, Held-to-Maturity, Accrued Interest, before Allowance for Credit Loss Securities held to maturity, Accrued interest receivable Repurchase of common stock Payments for Repurchase of Common Stock Repurchase of common stock Gain on transaction Unrealized Gain (Loss) on Derivatives Statement of Cash Flows [Abstract] Purchases With Credit Deterioration [Member] Purchases With Credit Deterioration [Member] Purchases with credit deterioration. Shares issued Shares issued for directors and employee stock plans Three SOFR Cap Agreement. Three SOFR Cap Agreement [Member] Represents amount of fair value disclosure of impaired loans. Impaired Loans Financing Receivable, Nonaccrual [Line Items] Derivative Asset, Statement of Financial Position [Extensible Enumeration] Interest rate swaps Interest Rate Swaps [Member] Accounts and Financing Receivables [Table] Earnings Per Share Earnings Per Share [Text Block] Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value Balance Balance Net income (loss) Net income (loss) Financing receivable originated checking accounts writeoffs annualized. Financing Receivable Originated Checking Accounts Writeoffs Annualized Financing receivable originated checking accounts writeoffs annualized Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases, (Sales), Issuances, (Settlements), Total Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases, (Sales), Issuances, (Settlements) Net purchases, sales, calls, and maturities Net change in market value of equity securities Net change in market value of equity securities Change in market value of equity securities Unrealized gains and (losses) recognized during the reporting period on securities still held at the reporting date Unrealized gains and (losses) recognized during the reporting period on securities still held at the reporting date Acquisition of assets from merger, net of cash Acquisition Of Assets From Merger, Net Of Cash Acquisition of assets from merger, net of cash. Intangible Assets, Net (Excluding Goodwill) Intangible Assets, Net (Excluding Goodwill), Total Intangible assets Derivative, Number of Instruments Held, Total Derivative, Number of Instruments Held Derivative, number of instruments held Provision for credit losses on loans Provision for Loan, Lease, and Other Losses Net Provision for (reversal of) credit losses expense Provision for (reversal of) credit losses Derivatives, Fair Value [Line Items] Loans [Member] Loans [Member] Assets [Abstract] Assets Federal Home Loan Bank stock Federal Home Loan Bank Stock Document Fiscal Year Focus Summary of Trouble Debt Restructurings (TDRs) Financing Receivable, Modified [Table Text Block] Other Assets [Member] Other Assets Concentration Risk Benchmark [Domain] Cash Provided by (Used in) Investing Activity, Including Discontinued Operation [Abstract] Cash flows from investing activities: Fixed coupon rate related to the derivative. cofs_DerivativeFixedCouponRate Derivative, fixed coupon rate Securities Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] Payment for shares retired after merger Payment for shares retired after merger. Fentura Financial shares retired after merger AAA [Member] A A A [Member] AAA. Business Combination, Separately Recognized Transactions, Revenues and Gains Recognized Fair value marks on acquired loans, which increased net interest income Cumulative amount of discontinued cash flow hedge Hedged Asset, Discontinued Fair Value Hedge, Cumulative Increase (Decrease) Deferred federal income tax (benefit)/expense Deferred Federal Income Tax Expense (Benefit) Net gains and (losses) recognized during the period Net gains and (losses) recognized during the period Net change in market value of equity securities Change in market value of equity securities Net gains and (losses) recognized during the period Payments to Acquire Assets, Investing Activities Issuance costs Derivative, Basis Spread on Variable Rate Derivative, Basis Spread on Variable Rate Subordinated Borrowing [Table] The amount of trust income. Trust income Other liabilities Other Liabilities, Total Other Liabilities Fair Value Disclosures [Abstract] Stock Issued During Period, Value, Acquisitions Merger with Fentura Financial, Inc., net of issuance costs Net Income (Loss) Available to Common Stockholders, Basic, Total Net Income (Loss) Available to Common Stockholders, Basic Net (loss) income Business Combination, Recognized Asset Acquired to Liability Assumed, Excess (Less) Net assets acquired Net assets acquired Long-term line of credit Long-Term Line of Credit, Total Long-Term Line of Credit Summary of Pro Forma Information Business Combination, Pro Forma Information [Table Text Block] Debt Instrument [Axis] Derivative Instrument [Axis] Represents net cash settlements received for derivative instruments. Derivative Instruments Net Cash Settlements Received Derivative Instruments, Net cash settlements received Derivative Instruments, Net settlements received Gain (Loss) on Sale of Loans and Leases, Total Gain (Loss) on Sale of Loans and Leases Gains on sales of loans Revenues, Total Revenues Total Revenues Cash Provided by (Used in) Investing Activity, Including Discontinued Operation Net cash provided by (used in) investing activities Sale of Interest Rate Swaps Sale of Interest Rate Swaps. Sale of interest rate swaps Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 Net transfers into Level 3 Pass [Member] Title of 12(b) Security Derivative and Hedging Activities Derivative Instruments and Hedging Activities Disclosure [Text Block] Interest Rate Contract, Hedged Items. Interest Rate Contract, Hedged Items [Member] Interest Expense [Member] Document Information [Table] Legal Entity [Axis] Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity, Increase (Decrease) in Operating Capital [Abstract] Net change in: Brokered deposits Brokered Deposits Brokered deposits. Schedule of Net Investment Hedges, Statements of Financial Performance and Financial Position, Location [Table Text Block] Schedule of Cumulative Basis Adjustments on Hedged Items Designated as Fair Value Hedges and Related Amortized Cost Time Deposits Time Deposits, Total Certificates of deposit (Note 9) Fair Value, Recurring and Nonrecurring [Table] Expected cash flows Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, Cash Flows Expected to be Collected at Acquisition The percent of stock outstanding to be purchased in stock repurchase program. cofs_StockRepurchaseProgramPercentOfStockOutstanding Stock Repurchase Program, Percent of Stock Outstanding Stock repurchase program, percent of stock outstanding Subordinated Debentures Subordinated Borrowings Disclosure [Text Block] Business Combination [Text Block] Business Combinations Measurement Frequency [Domain] Statement of Income Location, Balance [Axis] Change in Accounting Principle, Accounting Standards Update, Adoption Date Trading Symbol Represents information related to Employee Stock Purchase Plan. Employee Stock Purchase Plan [Member] Tabular disclosure of information about investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) and measured at amortized cost (held-to-maturity). Debt Securities, Available-for-Sale and Held-to-Maturity [Table Text Block] Schedule of Gross Unrealized Gains (Losses) on Investment Securities Available for Sale Recognized in Accumulated Other Comprehensive Income (loss) Fair Value Equity securities, at fair value (Note 2) Equity securities at fair value Subsequent Event Type [Axis] Change in net unrealized gain (loss) on cash flow hedge, Income tax benefit (expense) Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), before Reclassification, Tax Allowance for Credit Losses and Balances in Loan Portfolio Financing Receivable, Allowance for Credit Loss [Table Text Block] Fair value of investment in debt security measured at amortized cost (held-to-maturity). Securities held to maturity Cash Provided by (Used in) Financing Activity, Including Discontinued Operation [Abstract] Cash flows from financing activities: Fair Value, Recurring [Member] Subordinated Borrowings [Abstract] Stock Issued During Period, Value, Employee Stock Ownership Plan Repurchase of shares from Fentura Financial, Inc. ESOP Financing Receivable Portfolio Segment [Domain] August Two Thousand And Five Offering [Member] August two thousand and five offering member. August 2005 offering [Member] Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged Federal home loan bank, advances, general debt obligations, disclosures, collateral pledged Equity Components [Axis] Revolving Loans Financing Receivable, Revolving Schedule of Held to Maturity Securities with Unrealize Losses Debt Securities, Held-to-Maturity [Table Text Block] Equity Component [Domain] Federal Home Loan Bank Advances Maturing December 2027 [Member] Federal Home Loan Bank Advances maturing december 2027. Asset Class [Axis] The balance of financing receivables that were acquired with deteriorated credit quality. cofs_FinancingReceivableAcquiredWithDeterioratedCreditQuality Acquired with deteriorated credit quality Proceeds from issuance of subordinated long-term debt Proceeds from Issuance of Subordinated Long-Term Debt Federal Home Loan Bank Advances Maturing June 2026 [Member] Federal Home Loan Bank Advances Maturing June 2026. Amount, after tax, of adjustment from accumulated other comprehensive income (loss) stay unamortized on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) from transfer to investment in debt security measured at amortized cost (held-to-maturity). cofs_OciDebtSecuritiesAvailableforsaleTransferToHeldtomaturityAdjustmentFromAociUnamortizedAfterTax OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI Unamortized, After Tax Federal Home Loan Bank Advances, Disclosure [Text Block] Borrowings Schedule of Realized Gain (Loss) Schedule of Realized Gain (Loss) [Table Text Block] Other Noninterest Expense Other The amount of financial assets (as defined) recognized as of the originated loans. cofs_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOriginatedLoans Originated loans Held to Maturity Securities maturing within Less than 1 Year Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date, Rolling within One Year Other real estate owned, net Other Real Estate, Ending Balance Other Real Estate, Beginning Balance Other Real Estate, Total Other Real Estate Loan Restructuring Modification Name [Domain] Business Combination [Axis] Represents information related to community shores bank corp. Community Shores Bank Corp ("Community Shores") [Member] Community Shores Bank Corp ("Community Shores") [Member] Derivative Instrument, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net Derivative instrument, gain (loss) recognized in income, ineffective portion and amount excluded from effectiveness testing, net. Amount excluded from effectiveness testing recognized in earnings based on amortization approach Derivative, Fair Value, Net, Total Derivative, Fair Value, Net Fair Value Fair value Financing Receivable Interest Rate Reduction Financing receivable interest rate reduction. Interest rate reduction Other Unpaid Principal Balance with no related allowance recorded Pay Fixed Swap Agreements Expiring in 2032 [Member] Pay fixed swap agreements expiring in 2032. Loans, net Financing Receivable, after Allowance for Credit Loss U.S. Government and Federal Agency [Member] US Treasury and Government [Member] Statement of Comprehensive Income [Abstract] Maximum [Member] Maximum [Member] Debt Instrument, Increase (Decrease), Net Debt Instrument, Increase (Decrease), Net, Total Loans decreasing Designated as Hedging Instrument [Member] Not designated as hedges Current year-to-date gross write-offs, Grand Total Current year-to-date gross write-offs, Grand Total Writeoff of Notes Receivable Gross Writeoff of notes receivable gross. Statistical Measurement [Axis] Loans and Lease Receivable Outstanding Balance on PCD Loans Loans and lease receivable outstanding balance on PCD loans. Loan Balance Net Income (Loss) Available to Common Stockholders, Diluted, Total Net Income (Loss) Available to Common Stockholders, Diluted Net (loss) income Schedule of outstanding balance and related allowance on loans Schedule Of Outstanding Balance And Related Allowance On Loans Table [Text Block] Schedule of outstanding balance and related allowance on loans. Dividends [Axis] Principles of Consolidation Principles of Consolidation SOFR [Member] Secured Overnight Financing Rate (SOFR) [Member] Four SOFR cap Agreements. Four SOFR Cap Agreements [Member] Debt Securities, Available-for-Sale, Realized Gain Gross realized gains OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), before Tax Less: net unrealized (gains) losses on securities transferred from available-for-sale to held-to-maturity Less: net unrealized (gains) losses on securities transferred from available-for-sale to held-to-maturity Earnings on life insurance policies Securities Available For Sale [Member] Securities available for sale. Subordinated debentures Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Subordinated Debentures Business combination recognized identifiable assets acquired and liabilities assumed subordinated debentures. Preferred stock, authorized (in shares) Preferred Stock, Shares Authorized Average Recorded Investment Payment, Tax Withholding, Share-Based Payment Arrangement Share based compensation withholding obligation Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss Total, unrealized losses Total, unrealized losses Underlying Asset Class [Axis] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] The amount of liability arising from total deposits. cofs_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedTotalDeposits Total deposits Total deposits Interest-Bearing Deposit Liabilities, Total Interest-Bearing Deposit Liabilities Total interest-bearing deposits Deposits -- interest-bearing Accounts, Notes, Loans and Financing Receivable [Line Items] Revenue from Contracts with Customers Revenue from Contract with Customer [Text Block] Business Combination [Table] Individually evaluated for credit loss Financing Receivable, Allowance for Credit Loss, Individually Evaluated Loans allocated allowance for credit loss Entity Registrant Name Gross unrealized losses, available for sale Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, before Tax Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, before Tax, Total Customer Concentration Risk [Member] Single Customer The amount of liability arising from interest bearing deposits. cofs_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedInterestBearingDeposits Interest bearing deposits Reclassification, Type [Domain] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Income Tax Expense (Benefit), Total Income tax expense Income tax (benefit) expense Income tax expense (benefit) Net losses (gains) on sales of available for sale securities Debt Securities, Available-for-Sale, Gain (Loss) Debt Securities, Available-for-Sale, Gain (Loss), Total Measurement Frequency [Axis] Percentage of loans Percentage of Loan s and Leases Receivable Percentage of loan s and leases receivable. Stock Repurchased During Period, Value Shares repurchased Shares repurchased, cost Issuance of common stock Proceeds from Issuance of Common Stock Federal Home Loan Bank, Advances, General Debt Obligations, Amount of Available, Unused Funds Federal home loan bank, advances, general debt obligations, amount of available, unused funds Proceeds from Life Insurance Policy Proceeds from bank owned life insurance death benefits claim Financing receivable originated checking accounts writeoffs. Financing Receivable Originated Checking Accounts Writeoffs Financing receivable originated checking accounts writeoffs Entity Common Stock, Shares Outstanding State and Municipal [Member] US States and Political Subdivisions Debt Securities [Member] Accumulated other comprehensive loss, net Accumulated Other Comprehensive Income (Loss), Net of Tax, Total Accumulated Other Comprehensive Income (Loss), Net of Tax Commercial Real Estate Portfolio Segment [Member] Commercial Real Estate [Member] Payments to Acquire Property, Plant, and Equipment, Total Payments to Acquire Property, Plant, and Equipment Additions to premises and equipment Financing Receivable Originated Writeoffs in Fiscal Year Before Latest Fiscal Year Financing receivable originated writeoffs in fiscal year before latest fiscal year. Current year-to-date gross write-offs, Prior year Assets Total assets Consolidated total assets Total assets PCD Loans [Member] Performing Financial Instruments [Member] Class of Financing Receivable [Domain] Income Statement [Abstract] December Two Thousand And Four Offering [Member] December two thousand and four offering. December 2004 Offering [Member] A A [Member] AA [Member] A A [Member] Financing Receivable, Past Due [Line Items] Derivative, Contract Maturity Year Derivative, contract maturity year. Derivative, contract maturity year Unrealized losses Debt securities, held-to-maturity securities, unrealized loss position, accumulated loss. Debt securities, held-to-maturity securities, unrealized loss position, accumulated loss Equity Securities, FV-NI, Realized Gain (Loss), Total Equity Securities, FV-NI, Realized Gain (Loss) Less: Net gains and (losses) recognized during the period on securities sold Gain (Loss) on Sales of Loans, Net, Total Gains on sales of loans Business Combination, Reason for Business Combination Reason for business combination Debt Securities, Trading, and Equity Securities, FV-NI, Cost [Abstract] Shares issued (in shares) Shares issued for directors and employee stock plans (in shares) Amortized Cost Basis Of Collateral Dependent Loans [Member] Amortized Cost Basis Of Collateral Dependent Loans [Member] Business Combination, Integration-Related Cost, Expense Merger related expenses Business Combination [Abstract] Diluted earnings per common share Diluted (loss) earnings per common share Diluted earnings (loss) per share (Note 4) Common Stocks, Including Additional Paid in Capital Common stock and paid-in capital, no par value; shares authorized: 30,000,000; shares outstanding: 14,960,200 at March 31, 2026 and 15,000,939 at December 31, 2025 Total nonaccrual loans Nonaccrual loans Nonaccrual loans with ACL Subordinated Borrowing [Line Items] City Area Code Represents fair value portion of interest rate lock commitments. Interest rate lock commitments Financing Receivable Originated Writeoffs Two Years Before Latest Fiscal Year Financing receivable originated writeoffs two years before latest fiscal year. Current year-to-date gross write-offs, Two year prior Other comprehensive income (loss), net of tax Other comprehensive income (loss) Provision Financing Receivable, Credit Loss, Expense (Reversal) Provision for (reversal of) credit losses on loans Deposits Total deposits Commercial Real Estate [Member] Commercial Real Estate [Member] Intangible Assets, Finite-Lived, Policy [Policy Text Block] Customer List Intangible Line of Credit Facility, Maximum Borrowing Capacity Line of credit facility, maximum borrowing capacity Financial Asset, Past Due [Member] Business Combination [Line Items] Business Acquisition [Line Items] Fair Value Hedging [Member] Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss) Amount of total losses for the period included in earnings attributable to the change in unrealized gains (losses) relating to assets and liabilities still held at March 31, Interest and Fee Income, Loans and Leases, Total Loans, including fees Loans, including fees 30-89 days [Member] Maturity 30 to 90 Days [Member] Term Loans Total Term Loans Total Financing Receivable, Revolving, Converted to Term Loan Derivative Instrument, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net Derivative instrument, gain (loss) reclassified from accumulated OCI into income, effective portion, net. Amount of gain or (loss) reclassified from accumulated other comprehensive income into income Schedule of Revenue by Major Customers, by Reporting Segments [Table] Loans held for sale Loan, Held-for-Sale, Fair Value Disclosure Restricted stock units issued (in shares) Stock Issued During Period, Shares, Restricted Stock Award, Gross Business Combination, Consideration Transferred, Equity Interest Common stock purchased Premises and equipment, net Property, Plant and Equipment, Net, Total Property, Plant and Equipment, Net Goodwill Goodwill and Intangible Assets, Goodwill, Policy [Policy Text Block] Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents, Including Disposal Group and Discontinued Operations, Total Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Including Discontinued Operation Ending cash and cash equivalents Beginning cash and cash equivalents Retained Earnings [Member] Retained Earnings [Member] Represents information Directors' Stock Purchase Plan. Directors' Stock Purchase Plan [Member] Less: reclassification adjustment for net (gain) loss on cash flow hedge Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax Federal Home Loan Bank, Advance, Branch of FHLBank [Table] Share-Based Payment Arrangement, Noncash Expense, Total Share-Based Payment Arrangement, Noncash Expense Compensation expense on employee stock purchase plan, stock options, and restricted stock units Noninterest Income, Other Operating Income Noninterest income within the scope of other GAAP topics Represents information related to agriculture. Agricultural [Member] Agricultural [Member] Share Repurchase Program, Authorized, Number of Shares Stock repurchase program, number of shares authorized to be repurchased (in shares) Non-interest bearing deposits The amount of financial assets (as defined) recognized as non-interest bearing deposits. cofs_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoninterestBearingDeposits Less than 12 months, fair value Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months Less than 12 months, fair value Debt securities, held-to-maturity, continuous unrealized loss position, less than twelve months, fair value. Debt Securities, Held-to-Maturity, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value Fair Value, Inputs, Level 1 [Member] Cash Provided by (Used in) Financing Activity, Including Discontinued Operation Net provided by (used in) financing activities Collateral Held [Domain] Change in Accounting Principle, Accounting Standards Update, Adopted [true false] Related to debt. Subordinated Notes Due 2031 [Member] Subordinated Notes Due 2031 [Member] Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year Financing Receivable, Three year prior Borrowings Federal Home Loan Bank Borrowings, Fair Value Disclosure Derivative Contract [Domain] Current year-to-date gross write-offs, Term Loans Total Current year-to-date gross write-offs, Term Loans Total Wriyeoff of Financing Receivable Revolving Wriyeoff of financing receivable revolving. AOCI, Debt Securities, Available-for-Sale, Adjustment, after Tax Net unrealized gain US Government Corporations and Agencies Securities [Member] U. S. Government and Federal Agency [Member] Derivative, Remaining Maturity Remaining on the life of the swap Corporate [Member] Corporate Debt Securities [Member] Business Combination, Recognized Asset Acquired, Investment in Debt and Equity Securities, Current Securities available for sale Net interest income plus other income Business Acquisition, Pro Forma Net Interest Income Plus Other Income Business acquisition, pro forma net interest income plus other income. Financing Receivable, Troubled Debt Restructuring [Line Items] Entity Interactive Data Current Statement of Income Location, Balance [Domain] Federal Reserve Bank advances, general debt obligations, amount of available, unused funds. Federal Reserve Bank Advances, General Debt Obligations, Amount of Available, Unused Funds Federal reserve bank advances, eligible borrowing capacity Fair Value Measurement [Domain] Cash Provided by (Used in) Operating Activity, Including Discontinued Operation [Abstract] Cash flows from operating activities: Statement-Statement-Note-4-Earnings-Per-Share-Computation-Of-Basic-Earnings-Per-Share-And-Diluted-Earnings-Per-Share-Details Note 4 - Earnings Per Share - Computation of Basic Earnings Per Share and Diluted Earnings Per Share (Details) Business Combination, Consideration Transferred, Equity Interest, Share Issued, Number of Shares Number of shares issued (in shares) Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss, Total Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss Securities held to maturity, at amortized cost (Note 2) Loans by Type as Percentage of Portfolio Loans by Type as Percentage of Portfolio [Table Text Block] Loans by type as percentage of portfolio. Business Combination, Pro Forma Information [Abstract] Business Acquisition, Pro Forma Information [Abstract] Schedule of Reconciliation of Total Revenues, Profit or Loss, and Assets Segment, Reconciliation of Other Items from Segments to Consolidated [Table Text Block] Amount, before tax, of reclassification of amortization of net unrealized gain (loss) from accumulated other comprehensive income (AOCI) for derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness. cofs_OtherComprehensiveIncomeLossCashFlowHedgeAmortizationOfNetUnrealizedGainLossReclassificationBeforeTax Less: accretion of net unrealized (gains) losses included in net income Dividends declared, per share (in dollars per share) Dividends declared per share FETM Shares Retired [Member] FETM shares retired. FETM Shares Retired [Member] Entity Tax Identification Number Repurchase Agreements and Similar Transactions, Maturity Periods [Domain] Earnings on death benefit from bank-owned life insurance Earning on Death Benefit From Bank Owned Life Insurance Represents earnings on death benefit from bank owned life insurance. Current year-to-date gross write-offs, Revolving Loans Financing Receivable Writeoff Revolving Converted to Term Loan Financing receivable writeoff revolving converted to term loan. Long-Term Debt, Type [Axis] Maturities, prepayments and calls of securities available for sale Proceeds from Maturities, Prepayments and Calls of Debt Securities, Available-for-Sale Occupancy and equipment Less than 1 Year Fair value of investment in debt security, measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI), maturing in next rolling fiscal year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date. Less than 1 Year More than 10 Years Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), and equity security measured at fair value with change in fair value recognized in net income (FV-NI), with single maturity date and allocated without single maturity date, maturing after rolling tenth fiscal year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date. More than 10 Years Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings Total realized and unrealized (losses) gains included in noninterest income Derivative instrument cash payment Derivative Instrument Cash Payment Derivative instrument cash payment. Local Phone Number Financing Receivable, Held-for-Sale, Not Part of Disposal Group, after Valuation Allowance Loans held for sale Financing Receivable, Held-for-Sale, Not Part of Disposal Group, after Valuation Allowance, Ending Balance Financing Receivable, Held-for-Sale, Not Part of Disposal Group, after Valuation Allowance, Beginning Balance Represents information related to construction real estate. Construction Real Estate [Member] FDIC insurance Noninterest Expense Total noninterest expense Accounting Policies [Abstract] Disaggregation of Revenue [Line Items] Held to Maturity Securities maturing within 1 Year -5 Years Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date, Rolling after One through Five Years Sale of Securities to Pay Down Wholesale Funding Sale of securities to pay down wholesale funding. Sale of securities to pay down wholesale funding Residential Portfolio Segment [Member] Residential Real Estate [Member] External Credit Rating by Grouping [Domain] Salaries and benefits Represents fair value of accrued interest receivable. Accrued interest receivable cofs_DebtSecuritiesAvailableForSalePricePerPosition Per position price for debt securities classified as available-for-sale. Debt securities, available-for-sale, price per position Represents information related to commercial and industrial. Commercial and Industrial [Member] Commercial and Industrial [Member] Entity Emerging Growth Company Federal Home Loan Bank Advances [Member] Federal Home Loan Bank Advances [Member]