Accounts Receivable (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Accounts Receivable Additional Disclosures [Abstract]
Trade receivables, allowance for doubtful accounts	$ 700	$ 1,100
Accounts receivable, due date prior to applicable payroll date	1 day
Unbilled accounts receivable [Abstract]
Accrued worksite employee payroll cost	$ 215,214	161,917
Unbilled revenues	61,041	45,835
Customer prepayments	(21,256)	(17,037)
Unbilled accounts receivable	$ 254,999	$ 190,715