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Note 19 - Off-balance Sheet Activities (Details Textual)	12 Months Ended
	Dec. 31, 2020
Minimum [Member]
Interest Rate Commitments to Extend Credit	2.25%
Term Commitments to Extend Credit (Year)	1 year
Maximum [Member]
Interest Rate Commitments to Extend Credit	6.50%
Term Commitments to Extend Credit (Year)	30 years

X

- Definition

The interest rate for commitments to extend credit.

+ References

No definition available.

+ Details

Name:	cofs_InterestRateCommitmentsToExtendCredit
Namespace Prefix:	cofs_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The maturity term of commitments to extend credit.

+ References

No definition available.

+ Details

Name:	cofs_TermCommitmentsToExtendCredit
Namespace Prefix:	cofs_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: