Note 4 - Loans and Allowance for Loan Losses - Impaired Loans (Details) - USD ($)	3 Months Ended	12 Months Ended
Mar. 31, 2020	Dec. 31, 2019
Recorded investment, with no related allowance	$ 8,327,000	$ 3,740,000
Unpaid principal balance, with no related allowance	8,513,000	3,765,000
Average recorded investment, with no related allowance	8,393,000	3,985,000
Interest income recognized, with no related allowance	82,000	203,000
Recorded investment, with a related allowance	3,255,000	4,024,000
Related allowance	651,000	884,000
Unpaid principal balance, with a related allowance	3,255,000	4,175,000
Average recorded investment, with a related allowance	3,278,000	4,150,000
Interest income recognized, with a related allowance	43,000	194,000
Recorded investment	11,582,000	7,764,000
Unpaid principal balance	11,768,000	7,940,000
Average recorded investment	11,671,000	8,135,000
Interest income recognized	125,000	397,000
Commercial and Industrial Sector [Member]
Recorded investment, with no related allowance	792,000	133,000
Unpaid principal balance, with no related allowance	957,000	135,000
Average recorded investment, with no related allowance	812,000	156,000
Interest income recognized, with no related allowance	1,000	6,000
Recorded investment, with a related allowance	1,636,000	2,378,000
Related allowance	259,000	610,000
Unpaid principal balance, with a related allowance	1,636,000	2,529,000
Average recorded investment, with a related allowance	1,648,000	2,448,000
Interest income recognized, with a related allowance	24,000	113,000
Recorded investment	2,428,000	2,511,000
Unpaid principal balance	2,593,000	2,664,000
Average recorded investment	2,460,000	2,604,000
Interest income recognized	25,000	119,000
Residential Portfolio Segment [Member] \| Real Estate Sector [Member]
Recorded investment, with no related allowance	1,495,000	549,000
Unpaid principal balance, with no related allowance	1,500,000	553,000
Average recorded investment, with no related allowance	1,498,000	494,000
Interest income recognized, with no related allowance	17,000	20,000
Related allowance
Recorded investment	1,495,000	549,000
Unpaid principal balance	1,500,000	553,000
Average recorded investment	1,498,000	494,000
Interest income recognized	17,000	20,000
Commercial Portfolio Segment [Member] \| Real Estate Sector [Member]
Recorded investment, with no related allowance	5,187,000	3,058,000
Unpaid principal balance, with no related allowance	5,203,000	3,077,000
Average recorded investment, with no related allowance	5,230,000	3,335,000
Interest income recognized, with no related allowance	54,000	177,000
Recorded investment, with a related allowance	1,619,000	1,646,000
Related allowance	392,000	274,000
Unpaid principal balance, with a related allowance	1,619,000	1,646,000
Average recorded investment, with a related allowance	1,630,000	1,702,000
Interest income recognized, with a related allowance	19,000	81,000
Recorded investment	6,806,000	4,704,000
Unpaid principal balance	6,822,000	4,723,000
Average recorded investment	6,860,000	5,037,000
Interest income recognized	73,000	$ 258,000
Construction Portfolio Segment [Member] \| Real Estate Sector [Member]
Recorded investment, with no related allowance	853,000
Unpaid principal balance, with no related allowance	853,000
Average recorded investment, with no related allowance	853,000
Interest income recognized, with no related allowance	10,000
Related allowance
Recorded investment	853,000
Unpaid principal balance	853,000
Average recorded investment	853,000
Interest income recognized	$ 10,000

Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5212-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5258-111524

+ Details

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

- Definition

Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953550-111524

+ Details

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

- Definition

Amount of allowance for credit losses related to recorded investment.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableRelatedAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of average investment of impaired financing receivables for which there is no related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5258-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5212-111524

+ Details

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

- Definition

Amount of interest income, recognized on accrual basis, on financing receivables that are recognized as impaired, for which there is no related allowance for credit losses.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of unpaid principal balance of impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of average investment of impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of interest income, recognized on accrual basis, on financing receivables, for which there is a related allowance for credit losses and are recognized as impaired.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeAccrualMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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

- Definition

Unpaid principal balance of impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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

- Details

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

- 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_FinancingReceivablePortfolioSegmentAxis=cbfv_ConstructionPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: