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Workers' Compensation Claims - Additional Information (Detail) - USD ($)
 9 Months Ended 12 Months Ended
Jul. 01, 2021
Jun. 29, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Jun. 30, 2021
Jun. 30, 2020
Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Description of renewed agreement     The Company entered into a new arrangement for its insured program effective July 1, 2021 whereby third-party insurers assume all risk of loss for claims incurred after June 30, 2021. The agreement continues to June 30, 2022 and includes a renewal commitment through June 30, 2023. The arrangement allows for premium adjustments depending on overall portfolio performance. If claims develop favorably, BBSI can participate in the savings up to $20.0 million for a twelve-month policy period. If claims develop adversely, additional premium may be charged up to $7.5 million for a twelve-month policy period.          
Maximum savings amount for twelve month policy period, if claims develop favorably     $ 20,000,000.0          
Maximum additional premium charged amount for twelve month policy period, if claims develop adversely     7,500,000          
Balance in Chubb trust accounts   $ 115,700,000 293,100,000   $ 290,700,000      
Workers compensation liability     296,999,000 $ 347,348,000 357,746,000 $ 315,733,000 $ 336,504,000 $ 438,986,000
Workers compensation liability, current     78,225,000   102,040,000      
Collateral securities maintain to cover potential workers compensation claims losses     58,400,000   60,100,000      
Reserves incurred in excess of retention limits     2,300,000   2,900,000      
Other Assets [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Accrual for costs incurred in excess of retention limits offset by a receivable     $ 2,300,000   $ 2,900,000      
Insured Program [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Percentage of worker's compensation exposure covered     82.00%          
Worker's compensation risk claim maximum $ 3,000,000.0              
Worker's compensation risk claim maximum   $ 5,000,000.0            
Workers compensation liability     $ 53,100,000          
Workers compensation liability, current     $ 5,000,000.0          
Self Insured Programs [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Percentage of worker's compensation exposure covered     18.00%          
Worker's compensation risk claim maximum     $ 3,000,000.0 5,000,000.0        
Self Insured Programs [Member] | Maryland [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Reinsurance coverage     1,000,000.0 1,000,000.0        
Self Insured Programs [Member] | Colorado [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Reinsurance coverage     $ 2,000,000.0 $ 2,000,000.0