Workers' Compensation Claims - Additional Information (Detail) - USD ($)			6 Months Ended		12 Months Ended
	Jul. 01, 2021	Jun. 29, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020	Mar. 31, 2021	Mar. 31, 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	376,600,000		$ 290,700,000
Workers compensation liability			315,733,000	$ 336,504,000	357,746,000	$ 362,663,000	$ 445,310,000	$ 438,986,000
Workers compensation liability, current			86,047,000		102,040,000
Collateral securities maintain to cover potential workers compensation claims losses			58,400,000		60,100,000
Surety bonds and standby letters of credit			58,400,000
Reserves incurred in excess of retention limits			2,500,000		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			$ 2,500,000		$ 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		$ 5,000,000.0
Workers compensation liability			$ 53,100,000
Workers compensation liability, current			$ 5,000,000.0
Insured Program [Member] | Subsequent Event [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Worker's compensation risk claim maximum	$ 3,000,000.0
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