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Note 2 - Securities - Sales of Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Proceeds from sales of securities	$ 121,942	$ 178,913	$ 2,634
Gross realized gains	1,308	22	42
Gross realized losses	$ 0	$ 0	$ 8

X

- Definition

Amount of realized gain on investment in marketable security, including other-than-temporary impairment (OTTI).

+ References

No definition available.

+ Details

Name:	cofs_MarketableSecuritiesRealizedGain
Namespace Prefix:	cofs_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Amount of realized loss on investment in marketable security, including other-than-temporary impairment (OTTI).

+ References

No definition available.

+ Details

Name:	cofs_MarketableSecuritiesRealizedLoss
Namespace Prefix:	cofs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

The amount of cash inflow from the sale of securities.

+ References

No definition available.

+ Details

Name:	cofs_ProceedsFromSaleSecurities
Namespace Prefix:	cofs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration