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Summary of Operations and Significant Accounting Policies - Additional Information (Detail)
 12 Months Ended
Dec. 31, 2018
USD ($)
Branch
Segment
Company
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Basis Of Presentation Of Interim Period Statements [Line Items]        
Number Of Branch Offices | Branch 62      
Number of operating segment | Segment 1      
Revenue recognition professional employer service contracts term 1 year      
Professional employer service contract cancellation and termination period 30 days      
Payment term for staffing customers 30 days      
Number of wholly-owned insurance companies | Company 2      
Cash and cash equivalents, maturity term 3 months      
Allowance for doubtful accounts $ 533,000 $ 265,000 $ 78,000 $ 268,000
Number of reporting units | Segment 1      
Goodwill and intangible assets impairment $ 0      
Estimate of liability for unpaid safety incentives 29,200,000 28,500,000    
Income taxes paid (received) 1,800,000 9,900,000 4,200,000  
Restricted cash and restricted cash equivalents 105,331,000 $ 60,370,000 $ 290,562,000  
Other Accrued Liabilities [Member]        
Basis Of Presentation Of Interim Period Statements [Line Items]        
One-time customer Incentive 9,800,000      
Maximum [Member] | AICE [Member]        
Basis Of Presentation Of Interim Period Statements [Line Items]        
Reinsurance coverage $ 5,000,000      
Minimum [Member]        
Basis Of Presentation Of Interim Period Statements [Line Items]        
Likelihood of tax benefit being realized upon settlement 50.00%      
Minimum [Member] | Chubb Limited [Member]        
Basis Of Presentation Of Interim Period Statements [Line Items]        
Excess workers' compensation insurance coverage with Chubb $ 5,000,000      
California [Member] | Revenue [Member] | Geographic Concentration Risk [Member]        
Basis Of Presentation Of Interim Period Statements [Line Items]        
Percentage of revenue attributable to geographic area 79.00% 79.00% 78.00%  
Maryland [Member] | AICE [Member]        
Basis Of Presentation Of Interim Period Statements [Line Items]        
Reinsurance coverage $ 1,000,000      
Maryland [Member] | Minimum [Member] | Chubb Limited [Member]        
Basis Of Presentation Of Interim Period Statements [Line Items]        
Excess workers' compensation insurance coverage with Chubb 1,000,000      
Colorado [Member] | AICE [Member]        
Basis Of Presentation Of Interim Period Statements [Line Items]        
Reinsurance coverage $ 2,000,000      
Colorado [Member] | Chubb Limited [Member]        
Basis Of Presentation Of Interim Period Statements [Line Items]        
Excess workers' compensation insurance coverage with American Insurance Company statutory limits      
Colorado [Member] | Minimum [Member] | Chubb Limited [Member]        
Basis Of Presentation Of Interim Period Statements [Line Items]        
Excess workers' compensation insurance coverage with Chubb $ 2,000,000