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Basis of Presentation of Interim Period Statements - Additional Information (Detail) (USD $)
 3 Months Ended
Mar. 31, 2014
Mar. 31, 2013
Dec. 31, 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 $ 264,000   $ 242,000  
Estimated future liability for unsettled workers' compensation 120,135,000 77,212,000 112,444,000 70,564,000
Worker's compensation initial payment 20,000,000      
Estimate of liability for unpaid safety incentives 12,200,000   13,100,000  
Income taxes paid 1,200,000 5,900,000    
ACE [Member]
        
Basis Of Presentation Of Interim Period Statements [Line Items]        
Worker's Compensation Risk Claim Excess Value 5,000,000      
Barrett Business Services Inc [Member]
        
Basis Of Presentation Of Interim Period Statements [Line Items]        
Worker's Compensation Risk Claim 5,000,000      
Worker's compensation risk claim maximum 5,000,000      
Self-Insured States [Member] | ACE [Member]
        
Basis Of Presentation Of Interim Period Statements [Line Items]        
Excess workers' compensation insurance retention 5,000,000      
Maryland [Member] | ACE [Member]
        
Basis Of Presentation Of Interim Period Statements [Line Items]        
Excess workers' compensation insurance retention 1,000,000      
Colorado [Member] | ACE [Member]
        
Basis Of Presentation Of Interim Period Statements [Line Items]        
Excess workers' compensation insurance retention 2,000,000      
Excess workers' compensation insurance coverage with American Insurance Company Statutory Limits      
Minimum [Member] | ACE [Member]
        
Basis Of Presentation Of Interim Period Statements [Line Items]        
Excess workers' compensation insurance coverage with American Insurance Company 5,000,000      
Minimum [Member] | Maryland [Member] | ACE [Member]
        
Basis Of Presentation Of Interim Period Statements [Line Items]        
Excess workers' compensation insurance coverage with American Insurance Company 1,000,000      
Minimum [Member] | Colorado [Member] | ACE [Member]
        
Basis Of Presentation Of Interim Period Statements [Line Items]        
Excess workers' compensation insurance coverage with American Insurance Company 2,000,000      
Maximum [Member] | ACE [Member]
        
Basis Of Presentation Of Interim Period Statements [Line Items]        
Excess workers' compensation insurance coverage with American Insurance Company 15,000,000      
Maximum [Member] | Maryland [Member] | ACE [Member]
        
Basis Of Presentation Of Interim Period Statements [Line Items]        
Excess workers' compensation insurance coverage with American Insurance Company $ 25,000,000