Basis of Presentation of Interim Period Statements - Additional Information (Detail) (USD $)	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Mar. 31, 2014	Dec. 31, 2013	Mar. 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	$ 313,000			$ 242,000
Estimated future liability for unsettled workers' compensation	122,506,000	84,543,000	120,135,000	112,444,000	77,212,000	70,564,000
Balance in trust account	21,900,000		20,000,000
Restricted marketable securities and workers' compensation deposits	34,696,000			11,205,000
Estimate of liability for unpaid safety incentives	12,700,000			13,100,000
Income taxes paid	1,300,000	5,900,000
AICE [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
Worker's compensation risk claim excess value	5,000,000
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
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] | 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] | 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
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
Colorado [Member] | AICE [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
Excess workers' compensation insurance retention	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