Note 5 - Financial Instruments - Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Equity securities at fair value (Note 2)	$ 3,804	$ 2,896
Securities available for sale (Note 2)	722,627	574,787
Loans to other financial institutions	7,312	35,209
Deposits – noninterest-bearing	515,552	477,654
Deposits – interest-bearing	1,324,412	1,196,924
Fair Value, Inputs, Level 1 [Member]
Securities available for sale (Note 2)	0
Fair Value, Inputs, Level 2 [Member]
Securities available for sale (Note 2)	709,021
Fair Value, Inputs, Level 3 [Member]
Securities available for sale (Note 2)	13,606
Reported Value Measurement [Member]
Cash and cash equivalents	135,328	79,519
Equity securities at fair value (Note 2)	3,804	2,896
Securities available for sale (Note 2)	722,627	574,787
Federal Home Loan Bank and Federal Reserve Bank stock	8,004	8,004
Loans held for sale	18,736	12,921
Loans to other financial institutions	7,312	35,209
Loans, net	1,027,343	1,062,075
Accrued interest receivable	7,751	6,521
Interest rate lock commitments	1,756	842
Deposits – noninterest-bearing	515,552	477,654
Deposits – interest-bearing	1,324,412	1,196,924
Borrowings	3,484
Subordinated debentures	3,115	3,089
Accrued interest payable	129	183
Borrowings		9,327
Estimate of Fair Value Measurement [Member]
Cash and cash equivalents	135,328	79,519
Equity securities at fair value (Note 2)	3,804	2,896
Securities available for sale (Note 2)	722,627	574,787
Federal Home Loan Bank and Federal Reserve Bank stock	8,004	8,004
Loans held for sale	19,298	13,350
Loans to other financial institutions	7,312	35,209
Loans, net	1,024,203	1,057,786
Accrued interest receivable	7,751	6,521
Interest rate lock commitments	1,756	842
Deposits – noninterest-bearing	515,552	477,654
Deposits – interest-bearing	1,324,629	1,197,964
Borrowings	3,265
Subordinated debentures	2,918	3,089
Accrued interest payable	129	183
Borrowings		9,143
Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member]
Cash and cash equivalents	135,328	79,519
Equity securities at fair value (Note 2)	1,859	1,411
Securities available for sale (Note 2)	0	0
Federal Home Loan Bank and Federal Reserve Bank stock	0	0
Loans held for sale	0	0
Loans to other financial institutions	0	0
Loans, net	0	0
Accrued interest receivable	0	0
Interest rate lock commitments	0	0
Deposits – noninterest-bearing	0	0
Deposits – interest-bearing	0	0
Borrowings	0
Subordinated debentures	0	0
Accrued interest payable	0	0
Borrowings		0
Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 2 [Member]
Cash and cash equivalents	0	0
Equity securities at fair value (Note 2)	0	0
Securities available for sale (Note 2)	709,021	563,364
Federal Home Loan Bank and Federal Reserve Bank stock	8,004	8,004
Loans held for sale	19,298	13,350
Loans to other financial institutions	7,312	35,209
Loans, net	0	0
Accrued interest receivable	7,751	6,521
Interest rate lock commitments	1,756	842
Deposits – noninterest-bearing	515,552	477,654
Deposits – interest-bearing	1,324,629	1,197,964
Borrowings	3,265
Subordinated debentures	2,918	3,089
Accrued interest payable	129	183
Borrowings		9,143
Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 3 [Member]
Cash and cash equivalents	0	0
Equity securities at fair value (Note 2)	1,945	1,485
Securities available for sale (Note 2)	13,606	11,423
Federal Home Loan Bank and Federal Reserve Bank stock	0	0
Loans held for sale	0	0
Loans to other financial institutions	0	0
Loans, net	1,024,203	1,057,786
Accrued interest receivable	0	0
Interest rate lock commitments	0	0
Deposits – noninterest-bearing	0	0
Deposits – interest-bearing	0	0
Borrowings	0
Subordinated debentures	0	0
Accrued interest payable	$ 0	0
Borrowings		$ 0

Fair value portion of accrued interest payable.

No definition available.

Name:	cofs_AccruedInterestPayableFairValueDisclosure
Namespace Prefix:	cofs_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents fair value of accrued interest receivable.

+ References

No definition available.

+ Details

Name:	cofs_AccruedInterestReceivableFairValueDisclosure
Namespace Prefix:	cofs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the fair value portion of Federal Home Loan Bank Borrowings and Term Loans.

+ References

No definition available.

+ Details

Name:	cofs_FederalHomeLoanBankBorrowingsAndTermLoansFairValueDisclosure
Namespace Prefix:	cofs_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents fair value portion of interest rate lock commitments.

+ References

No definition available.

+ Details

Name:	cofs_InterestRateLockCommitmentsFairValueDisclosure
Namespace Prefix:	cofs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount related to loans to other financial institutions.

+ References

No definition available.

+ Details

Name:	cofs_LoansAndLeasesReceivableOtherFinancialInstitution
Namespace Prefix:	cofs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

No definition available.

+ Details

Name:	us-gaap_CashAndCashEquivalentsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

+ Details

Name:	us-gaap_EquitySecuritiesFvNi
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of subordinated debt obligations including, but not limited to, subordinated loan, subordinated bond, subordinated debenture or junior debt.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value portion of borrowing from Federal Home Loan Bank (FHLBank).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.13(3)(a))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

Name:	us-gaap_FederalHomeLoanBankBorrowingsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of Federal Home Loan Bank (FHLB) and Federal Reserve Bank stock.

+ References

No definition available.

+ Details

Name:	us-gaap_FederalHomeLoanBankStockAndFederalReserveBankStock
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The aggregate of all domestic and foreign interest-bearing deposit liabilities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.12)
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

Name:	us-gaap_InterestBearingDepositLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables.

+ References

No definition available.

+ Details

Name:	us-gaap_LoansHeldForSaleFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-URI http://asc.fasb.org/subtopic&trid=2196772

+ Details

Name:	us-gaap_LoansReceivableFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The aggregate amount of all domestic and foreign noninterest-bearing deposits liabilities held by the entity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.12)
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

Name:	us-gaap_NoninterestBearingDepositLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_CarryingReportedAmountFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: