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Repurchase Agreements (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Disclosure of Repurchase Agreements [Abstract]
Average balance during the year	$ 1,412
Average interest rate during the year	0.05%
Maximum month end balance during the year	$ 7,148

X

- Definition

The calculated average balance during the year.

+ References

No definition available.

+ Details

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

X

- Definition

The calculated average interest rate paid during the year.

+ References

No definition available.

+ Details

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

X

- Definition

The maximum month end balance of securities sold under repurchase agreements.

+ References

No definition available.

+ Details

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

X

- References

No definition available.

+ Details

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