Accounts Receivable (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Accounts Receivable Additional Disclosures [Abstract]
Trade receivables, allowance for doubtful accounts	$ 600	$ 700
Accounts receivable, due date prior to applicable payroll date	1 day
Unbilled accounts receivable [Abstract]
Accrued worksite employee payroll cost	$ 267,402	215,214
Unbilled revenues	74,632	61,041
Customer prepayments	(23,603)	(21,256)
Unbilled accounts receivable	$ 318,431	$ 254,999