Note 6 - Loans and Related Allowance for Loan Loss - Loans Classified as TDRs (Details) $ in Thousands	3 Months Ended
Mar. 31, 2019 USD ($)	Mar. 31, 2018
Residential Portfolio Segment [Member]
Number of contracts		1
Originated Loans [Member]
Number of contracts	1
Pre-modification outstanding recorded investment	$ 61
Post-modification outstanding recorded investment	61
Related allowance
Originated Loans [Member] \| Residential Portfolio Segment [Member] \| Real Estate Sector [Member]
Number of contracts	1
Pre-modification outstanding recorded investment	$ 61
Post-modification outstanding recorded investment	61
Related allowance

The amount of reserve to cover probable credit losses related to financing receivables that were modified by troubled debt restructurings during the reporting period and were individually evaluated for impairment

No definition available.

+ Details

Name:	cbfv_AllowanceForLoanLossesOnTDRs
Namespace Prefix:	cbfv_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953676-111524

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after troubled debt restructuring, of modified financing receivable.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount, before troubled debt restructuring, of financing receivable to be modified.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=cbfv_OriginatedLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_EquitySecuritiesByIndustryAxis=us-gaap_RealEstateSectorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: