Basis of Presentation of Interim Period Statements - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2018 USD ($) Company	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Revenue recognition professional employer service contracts term	1 year
Professional employer service contract cancellation and termination period	30 days
Payment term for staffing customers	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	482,000		265,000
Estimate of liability for unpaid safety incentives	28,600,000		28,500,000
Income taxes paid	100,000	$ 2,500,000
Interest paid	1,800,000
Restricted cash and restricted cash equivalents	$ 70,149,000	$ 50,200,000	$ 60,370,000	$ 290,600,000