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Benefit Plans - Additional Information (Detail) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Compensation And Retirement Disclosure [Abstract] |  |  |
Discretionary Company contributions | $ 0 | $ 0 | $ 0
Employer matching contribution | 1,900,000 | 1,600,000 | $ 1,500,000
Deferred compensation plan liability | $ 3,500,000 | $ 1,100,000 |

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- Definition Defined contribution plan employer matching contribution amount. + References No definition available. + Details
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Namespace Prefix: | bbsi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- 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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- Definition Aggregate carrying value as of the balance sheet date of the liabilities for all deferred compensation arrangements payable beyond one year (or the operating cycle, if longer). + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount of discretionary contributions made by an employer to a defined contribution plan. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
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Period Type: | duration