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Consolidated Statements of Changes in Shareholders' Equity (Parentheticals)	12 Months Ended
	Dec. 31, 2018 $ / shares
Cash Dividend [Member]
Dividends declared, per share (in dollars per share)	$ 0.71
Stock dividend declared	5.00%
Dividends declared, per share (in dollars per share)	$ 0.71

X

- Definition

Percentage of shares of common and preferred stock issued as dividends during the period. Excludes stock splits.

+ References

No definition available.

+ Details

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Namespace Prefix:	cofs_
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Balance Type:	na
Period Type:	duration

X

- Definition

Aggregate dividends declared during the period for each share of common stock outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	na
Period Type:	duration

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type: