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Note 19 - Off-balance Sheet Activities (Details Textual) | 12 Months Ended
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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: |