LOANS AND ALLOWANCE FOR LOAN LOSSES (Details 1) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	$ 401,916	$ 398,785
Nonaccrual past due loans	1,271	1,096
Agricultural [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	44,397	48,464
Nonaccrual past due loans	413	423
Agricultural [Member] \| Risk ratings 1 and 2 [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	12,331	14,813
Agricultural [Member] \| Risk rating 3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	22,299	22,721
Agricultural [Member] \| Risk rating 4 [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	8,952	10,199
Agricultural [Member] \| Risk rating 5 [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	403	308
Agricultural [Member] \| Risk rating 6 [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	412	423
Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	92,157	104,386
Commercial and Industrial [Member] \| Risk ratings 1 and 2 [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	12,665	13,491
Commercial and Industrial [Member] \| Risk rating 3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	53,958	63,366
Commercial and Industrial [Member] \| Risk rating 4 [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	20,944	26,943
Commercial and Industrial [Member] \| Risk rating 5 [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	4,500	491
Commercial and Industrial [Member] \| Risk rating 6 [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	90	95
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	138,072	123,487
Nonaccrual past due loans	130	222
Commercial Real Estate [Member] \| Risk ratings 1 and 2 [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	8,126	8,227
Commercial Real Estate [Member] \| Risk rating 3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	93,626	78,868
Commercial Real Estate [Member] \| Risk rating 4 [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	32,485	33,429
Commercial Real Estate [Member] \| Risk rating 5 [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	2,554	1,533
Commercial Real Estate [Member] \| Risk rating 6 [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	1,281	1,430
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	24,363	24,513
Nonaccrual past due loans	5	15
Consumer [Member] \| Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	24,358	24,497
Consumer [Member] \| Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans		1
Construction Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	7,230	6,613
Nonaccrual past due loans	268
Construction Real Estate [Member] \| Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	6,962	6,613
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	95,697	91,322
Nonaccrual past due loans	455	436
Residential Real Estate [Member] \| Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans	$ 95,242	90,629
Residential Real Estate [Member] \| Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Risk rated loans		$ 257

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

+ Details

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

- Definition

Recorded investment in financing receivables that are on nonaccrual status as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5111-111524

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

+ Details

Name:	us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cofs_AgriculturalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_PassMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=cofs_Pass1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=cofs_Pass2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_SpecialMentionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_SubstandardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cofs_FinancingReceivableCommercialAndIndustrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_PerformingFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_NonperformingFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: