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Deposits - Additional Information (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Statistical Disclosure for Banks [Abstract]
Time deposits, $250,000 or more	$ 210.5	$ 199.7
Interest-bearing and noninterest-bearing deposit, brokered	36.5	23.4
Certificates of deposit issued through CDARS	$ 41.0	$ 36.2

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- Definition

Certificates of deposit issued through the Certificate of Deposit Account Registry Service (CDARS).

+ References

No definition available.

+ Details

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

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- Definition

Amount of interest-bearing and noninterest-bearing third-party deposits placed by or through the assistance of a deposit broker, also known as brokered deposit liabilities.

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No definition available.

+ Details

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

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- Definition

Amount of time deposit liabilities, including certificates of deposit, in denominations of $250,000 or more.

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No definition available.

+ Details

Name:	cofs_TimeDeposits250000OrMore
Namespace Prefix:	cofs_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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- References

No definition available.

+ Details

Name:	srt_StatisticalDisclosureForBanksAbstract
Namespace Prefix:	srt_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration