Quarterly report pursuant to Section 13 or 15(d)

Workers' Compensation Claims - Additional Information (Detail)

v3.8.0.1
Workers' Compensation Claims - Additional Information (Detail) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Balance in Chubb trust account $ 399,700,000 $ 380,600,000    
Amount remitted to Chubb 12,100,000      
Specified investment balances or other financial instruments 84,700,000 96,800,000    
Surety bonds and standby letters of credit 84,700,000      
Estimated future liability for unsettled workers' compensation 378,874,000 363,517,000 $ 326,233,000 $ 312,537,000
Reserves incurred in excess of retention limits 2,900,000 3,000,000    
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 2,900,000 $ 3,000,000    
California [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Surety bonds and standby letters of credit 70,200,000      
Barrett Business Services Inc [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Worker's compensation risk claim maximum $ 5,000,000