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CONSOLIDATED STATEMENTS OF INCOME (Unaudited) (Parenthetical)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Stock dividend paid (percent)	5.00%	5.00%

X

- Definition

The stock dividend paid, represented as a percentage.

+ References

No definition available.

+ Details

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

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- References

No definition available.

+ Details

Name:	us-gaap_IncomeStatementAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration