Note 4 - Loans and Allowance for Loan Losses - Loans Summarized by Individually Evaluated for Impairment and Collectively Evaluated for Potential Losses (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Individually Evaluated for Impairment	$ 11,582	$ 7,764
Collectively Evaluated for Potential Impairment	963,068	944,732
Loans and Leases Receivable, Gross, Total	974,650	952,496
Residential Portfolio Segment [Member] \| Real Estate Sector [Member]
Individually Evaluated for Impairment	1,495	549
Collectively Evaluated for Potential Impairment	345,369	347,217
Loans and Leases Receivable, Gross, Total	346,864	347,766
Commercial Portfolio Segment [Member] \| Real Estate Sector [Member]
Individually Evaluated for Impairment	6,806	4,704
Collectively Evaluated for Potential Impairment	347,568	346,656
Loans and Leases Receivable, Gross, Total	354,374	351,360
Commercial Portfolio Segment [Member] \| Commercial and Industrial Sector [Member]
Individually Evaluated for Impairment	2,428	2,511
Collectively Evaluated for Potential Impairment	78,293	83,075
Loans and Leases Receivable, Gross, Total	80,721	85,586
Construction Portfolio Segment [Member] \| Real Estate Sector [Member]
Individually Evaluated for Impairment	853
Collectively Evaluated for Potential Impairment	49,164	35,605
Loans and Leases Receivable, Gross, Total	50,017	35,605
Consumer Portfolio Segment [Member]
Individually Evaluated for Impairment
Collectively Evaluated for Potential Impairment	121,494	113,637
Loans and Leases Receivable, Gross, Total	121,494	113,637
Other Portfolio Segment [Member]
Individually Evaluated for Impairment
Collectively Evaluated for Potential Impairment	21,180	18,542
Loans and Leases Receivable, Gross, Total	$ 21,180	$ 18,542

The balance of financing receivables that were collectively evaluated for impairment.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953423-111524

+ Details

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

- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=121593590&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_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
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:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=cbfv_ConstructionPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=cbfv_OtherPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: