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Basis of Presentation of Interim Period Statements - Additional Information (Detail) (USD $)
 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Jun. 30, 2013
Dec. 31, 2012
Jun. 30, 2012
Dec. 31, 2011
Basis Of Presentation Of Interim Period Statements [Line Items]            
Professional employer service contract cancellation and termination period 30 days          
Allowance for doubtful accounts $ 427,000     $ 381,000    
Estimated future liability for unsettled workers' compensation 95,980,000 61,628,000 84,543,000 70,564,000 55,765,000 51,193,000
Estimate of liability for unpaid safety incentives 11,900,000     9,800,000    
Income taxes paid 6,100,000 246,000        
Minimum [Member]
            
Basis Of Presentation Of Interim Period Statements [Line Items]            
Revenue recognition professional employer services contracts term 1 year          
Minimum [Member] | AICE [Member]
            
Basis Of Presentation Of Interim Period Statements [Line Items]            
Excess workers' compensation insurance coverage with American Insurance Company 5,000,000          
Maximum [Member] | AICE [Member]
            
Basis Of Presentation Of Interim Period Statements [Line Items]            
Excess workers' compensation insurance coverage with American Insurance Company 15,000,000          
Self-Insured States [Member] | AICE [Member]
            
Basis Of Presentation Of Interim Period Statements [Line Items]            
Excess workers' compensation insurance retention 5,000,000          
Maryland [Member] | AICE [Member]
            
Basis Of Presentation Of Interim Period Statements [Line Items]            
Excess workers' compensation insurance retention 1,000,000          
Maryland [Member] | Minimum [Member] | AICE [Member]
            
Basis Of Presentation Of Interim Period Statements [Line Items]            
Excess workers' compensation insurance coverage with American Insurance Company 1,000,000          
Maryland [Member] | Maximum [Member] | AICE [Member]
            
Basis Of Presentation Of Interim Period Statements [Line Items]            
Excess workers' compensation insurance coverage with American Insurance Company 25,000,000          
Colorado [Member] | AICE [Member]
            
Basis Of Presentation Of Interim Period Statements [Line Items]            
Excess workers' compensation insurance retention 500,000          
Colorado [Member] | Minimum [Member] | AICE [Member]
            
Basis Of Presentation Of Interim Period Statements [Line Items]            
Excess workers' compensation insurance coverage with American Insurance Company $ 500,000          
Colorado [Member] | Maximum [Member] | AICE [Member]
            
Basis Of Presentation Of Interim Period Statements [Line Items]            
Excess workers' compensation insurance coverage with American Insurance Company Statutory Limits