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Basis of Presentation of Interim Period Statements - Additional Information (Detail) (USD $)
 3 Months Ended 9 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Jun. 30, 2013
Dec. 31, 2012
Basis Of Presentation Of Interim Period Statements [Line Items]                  
Revenue recognition professional employer services contracts term     1 year            
Professional employer service contract cancellation and termination period     30 days            
Allowance for doubtful accounts $ 313,000   $ 313,000       $ 242,000    
Estimated future liability for unsettled workers' compensation 208,343,000 95,980,000 208,343,000 95,980,000 122,506,000   112,444,000 84,543,000 70,564,000
Total workers' compensation claims expense 101,159,000 22,398,000 140,162,000 57,513,000          
Change in estimate workers' compensation claims expense relating to prior period 61,283,000 5,002,000 66,387,000 12,688,000          
Balance in trust account 31,100,000   31,100,000     20,000,000      
Restricted marketable securities and workers' compensation deposits 43,821,000   43,821,000       11,205,000    
Estimate of liability for unpaid safety incentives 14,100,000   14,100,000       13,100,000    
Income taxes paid     3,000,000 6,100,000          
ACE Group [Member] | Minimum [Member]
                  
Basis Of Presentation Of Interim Period Statements [Line Items]                  
Excess workers' compensation insurance coverage with ACE group 5,000,000   5,000,000            
ACE Group [Member] | Maximum [Member]
                  
Basis Of Presentation Of Interim Period Statements [Line Items]                  
Excess workers' compensation insurance coverage with ACE group 15,000,000   15,000,000            
Barrett Business Services Inc [Member]
                  
Basis Of Presentation Of Interim Period Statements [Line Items]                  
Worker's compensation risk claim 5,000,000   5,000,000            
Worker's compensation risk claim maximum     5,000,000            
Self-Insured States [Member] | AICE [Member]
                  
Basis Of Presentation Of Interim Period Statements [Line Items]                  
Excess workers' compensation insurance retention 5,000,000   5,000,000            
Maryland [Member] | AICE [Member]
                  
Basis Of Presentation Of Interim Period Statements [Line Items]                  
Excess workers' compensation insurance retention 1,000,000   1,000,000            
Maryland [Member] | ACE Group [Member] | Minimum [Member]
                  
Basis Of Presentation Of Interim Period Statements [Line Items]                  
Excess workers' compensation insurance coverage with ACE group 1,000,000   1,000,000            
Maryland [Member] | ACE Group [Member] | Maximum [Member]
                  
Basis Of Presentation Of Interim Period Statements [Line Items]                  
Excess workers' compensation insurance coverage with ACE group 25,000,000   25,000,000            
Colorado [Member] | AICE [Member]
                  
Basis Of Presentation Of Interim Period Statements [Line Items]                  
Excess workers' compensation insurance retention 2,000,000   2,000,000            
Colorado [Member] | ACE Group [Member]
                  
Basis Of Presentation Of Interim Period Statements [Line Items]                  
Excess workers' compensation insurance coverage with American Insurance Company     Statutory Limits            
Colorado [Member] | ACE Group [Member] | Minimum [Member]
                  
Basis Of Presentation Of Interim Period Statements [Line Items]                  
Excess workers' compensation insurance coverage with ACE group $ 2,000,000   $ 2,000,000