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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