Basis of Presentation of Interim Period Statements - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013
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	$ 241,000		$ 291,000
Balance in trust account	78,000,000		50,100,000
Estimated future liability for unsettled workers' compensation	222,265,000	120,135,000	215,982,000	112,444,000
Estimate of liability for unpaid safety incentives	15,500,000		14,200,000
Income taxes Received	1,200,000
Income taxes paid		1,200,000
AICE [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
Worker's compensation risk claim excess value	5,000,000
AICE [Member] | Self-Insured States [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
Excess workers' compensation insurance retention	5,000,000
AICE [Member] | Maryland [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
Excess workers' compensation insurance retention	1,000,000
AICE [Member] | Colorado [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
Excess workers' compensation insurance retention	2,000,000
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
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
ACE Group [Member] | Maryland [Member] | Minimum [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
Excess workers' compensation insurance coverage with ACE group	1,000,000
ACE Group [Member] | Maryland [Member] | Maximum [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
Excess workers' compensation insurance coverage with ACE group	25,000,000
ACE Group [Member] | Colorado [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
Excess workers' compensation insurance coverage with American Insurance Company	Statutory limits
ACE Group [Member] | Colorado [Member] | Minimum [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
Excess workers' compensation insurance coverage with ACE group	2,000,000
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