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Fair Value Measurement - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Mortgage Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Average contractual maturity period	17 years	17 years

X

- Definition

Debt securities available for sale, average contractual maturity period.

+ References

No definition available.

+ Details

Name:	bbsi_DebtSecuritiesAvailableForSaleAverageContractualMaturityPeriod
Namespace Prefix:	bbsi_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

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.

+ References

No definition available.

+ Details

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

X

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_MortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: