Note 5 - Financial Instruments - Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Equity securities at fair value (Note 2)	$ 2,905	$ 2,851
Securities available for sale (Note 2)	372,525	339,579
Loans to other financial institutions	49,895	51,048
Deposits – noninterest-bearing	392,086	287,460
Deposits – interest-bearing	932,222	867,142
Reported Value Measurement [Member]
Cash and cash equivalents	66,791	59,558
Equity securities at fair value (Note 2)	2,905	2,851
Securities available for sale (Note 2)	372,525	339,579
Reserve Bank stock	6,471	6,458
Loans held for sale	10,860	3,095
Loans to other financial institutions	49,895	51,048
Loans, net	902,243	797,991
Accrued interest receivable	5,424	3,965
Interest rate lock commitments	1,084	68
Deposits – noninterest-bearing	392,086	287,460
Deposits – interest-bearing	932,222	867,142
Borrowings	10,179
Accrued interest payable	246	411
Federal Home Loan Bank advances		33,198
Estimate of Fair Value Measurement [Member]
Cash and cash equivalents	66,791	59,558
Equity securities at fair value (Note 2)	2,905	2,851
Securities available for sale (Note 2)	372,525	339,579
Reserve Bank stock	6,471	6,458
Loans held for sale	11,186	3,134
Loans to other financial institutions	49,895	51,048
Loans, net	899,773	793,270
Accrued interest receivable	5,424	3,965
Interest rate lock commitments	1,084	68
Deposits – noninterest-bearing	392,086	287,460
Deposits – interest-bearing	933,484	867,154
Borrowings	10,229
Accrued interest payable	246	411
Federal Home Loan Bank advances		33,243
Estimate of Fair Value Measurement [Member] \| Fair Value, Inputs, Level 1 [Member]
Cash and cash equivalents	66,791	59,558
Equity securities at fair value (Note 2)	1,425	1,379
Securities available for sale (Note 2)	0	0
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
Accrued interest payable	0	0
Federal Home Loan Bank advances		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)	360,380	327,212
Reserve Bank stock	6,471	6,458
Loans held for sale	11,186	3,134
Loans to other financial institutions	49,895	51,048
Loans, net	0	0
Accrued interest receivable	5,424	3,965
Interest rate lock commitments	1,084	68
Deposits – noninterest-bearing	392,086	287,460
Deposits – interest-bearing	933,484	867,154
Borrowings	10,229
Accrued interest payable	246	411
Federal Home Loan Bank advances		33,243
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)	1,480	1,472
Securities available for sale (Note 2)	12,145	12,367
Reserve Bank stock	0	0
Loans held for sale	0	0
Loans to other financial institutions	0	0
Loans, net	899,773	793,270
Accrued interest receivable	0	0
Interest rate lock commitments	0	0
Deposits – noninterest-bearing	0	0
Deposits – interest-bearing	0	0
Borrowings	0
Accrued interest payable	$ 0	0
Federal Home Loan Bank advances		$ 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 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_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:

- 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: