Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Cash and Cash Equivalents [Line Items]
Overnight Holdings	$ 366,128	$ 311,158
Short-term Investments	78,088	77,492
Cash	25,809	33,207
Drafts Payable	(17,490)	(34,303)
Cash, Cash Equivalents, and Short-term Investments	452,535	387,554
Payroll Withholdings Included in Cash Balance	279,600	224,500
Client Prepayments Included in Cash Balance	32,400	34,200
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings	0	0
Short-term Investments	53,599	60,781
Cash	0	0
Drafts Payable	0	0
Cash, Cash Equivalents, and Short-term Investments	53,599	60,781
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings	366,128	311,158
Short-term Investments	24,489	16,711
Cash	25,809	33,207
Drafts Payable	(17,490)	(34,303)
Cash, Cash Equivalents, and Short-term Investments	$ 398,936	$ 326,773