Summary of Operations and Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Branch Segment Company	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Basis Of Presentation Of Interim Period Statements [Line Items]
Number Of Branch Offices | Branch	58
Number of operating segment | Segment	1
Revenue recognition professional employer services contracts term	1 year
Professional employer service contract cancellation and termination period	30 days
Number of wholly-owned insurance companies | Company	2
Cash and cash equivalents, maturity term	3 months
Trading securities	$ 0	$ 0
Allowance for doubtful accounts	$ 265,000	78,000	$ 268,000	$ 291,000
Number of reporting units | Segment	1
Goodwill and intangible assets impairment	$ 0
Estimate of liability for unpaid safety incentives	28,500,000	24,800,000
Income taxes paid (received)	9,900,000	4,200,000	$ (4,100,000)
Deferred income taxes		$ 25,200,000
Restricted cash and investments	70,600,000
Maximum [Member] | AICE [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
Reinsurance coverage	$ 5,000,000
Minimum [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
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	79.00%	78.00%	78.00%
Maryland [Member] | AICE [Member]
Basis Of Presentation Of Interim Period Statements [Line Items]
Reinsurance coverage	$ 1,000,000
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]
Reinsurance coverage	$ 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