Accounts Receivable (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Accounts Receivable Additional Disclosures [Abstract]
Accounts Receivable, Net, Current	$ 12,731	$ 10,015
Unbilled accounts receivable	448,088	385,567
Other Receivables	4,960	5,041
Accounts receivable, net	465,779	400,623
Allowance for Doubtful Accounts Receivable, Current	$ 900	1,000
Accounts receivable, due date prior to applicable payroll date	1 day
Unbilled accounts receivable [Abstract]
Accrued worksite employee payroll cost	$ 401,859	329,979
Unbilled revenues	105,841	89,765
Client Prepayments Included in Cash Balance	(59,612)	(34,177)
Unbilled accounts receivable	$ 448,088	$ 385,567