Accounts Receivable (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Accounts Receivable Additional Disclosures [Abstract]
Accounts Receivable, Net, Current	$ 10,015	$ 12,292
Unbilled accounts receivable	385,567	318,431
Other Receivables	5,041	3,258
Accounts receivable, net	400,623	333,981
Allowance for Doubtful Accounts Receivable, Current	$ 1,000	600
Accounts receivable, due date prior to applicable payroll date	1 day
Unbilled accounts receivable [Abstract]
Accrued worksite employee payroll cost	$ 329,979	267,402
Unbilled revenues	89,765	74,632
Client Prepayments Included in Cash Balance	34,177	23,603
Unbilled accounts receivable	$ 385,567	$ 318,431