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v3.20.4

Fair Value Measurement - Additional Information (Detail) | 12 Months Ended
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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

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- Definition Debt securities available for sale, average contractual maturity period. + References No definition available. + Details
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Namespace Prefix: | bbsi_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |