Summary of Operations and Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016 USD ($) Branch Segment	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Basis Of Presentation Of Interim Period Statements [Line Items]
Number Of Branch Offices | Branch	57
Revenue recognition professional employer services contracts term	1 year
Professional employer service contract cancellation and termination period	30 days
Cash and cash equivalents, maturity term	3 months
Trading securities	$ 0	$ 0
Number of reporting units | Segment	1
Goodwill and intangible assets impairment	$ 0
Estimate of liability for unpaid safety incentives	24,800,000	21,300,000
Income taxes paid (received)	4,200,000	(4,100,000)	$ 3,300,000
Deferred income taxes	$ 25,242,000	$ 20,941,000
Maximum [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
Restricted cash and investments maturity period	2 years
Maximum [Member] | AICE [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
Workers Compensation Coverage Per Occurrence	$ 5,000,000
Maximum [Member] | Chubb Limited [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
Excess workers' compensation insurance coverage with Chubb	$ 15,000,000
Minimum [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
Restricted cash and investments maturity period	180 days
Likelihood of tax benefit being realized upon settlement	50.00%
Minimum [Member] | Chubb Limited [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
Excess workers' compensation insurance coverage with Chubb	$ 5,000,000
California [Member] | Revenue [Member] | Geographic Concentration Risk [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
Percentage of revenue attributable to geographic area	78.00%	78.00%	77.00%
Maryland [Member] | AICE [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
Excess workers' compensation insurance coverage with Chubb	$ 1,000,000
Maryland [Member] | Maximum [Member] | Chubb Limited [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
Excess workers' compensation insurance coverage with Chubb	25,000,000
Maryland [Member] | Minimum [Member] | Chubb Limited [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
Excess workers' compensation insurance coverage with Chubb	1,000,000
Colorado [Member] | AICE [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
Excess workers' compensation insurance coverage with Chubb	$ 2,000,000
Colorado [Member] | Chubb Limited [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
Excess workers' compensation insurance coverage with American Insurance Company	statutory limits
Colorado [Member] | Minimum [Member] | Chubb Limited [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
Excess workers' compensation insurance coverage with Chubb	$ 2,000,000