Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Cash and Cash Equivalents [Line Items]
Overnight Holdings	$ 432,607	$ 503,221
Short-term Investments	82,570	82,521
Cash	23,105	33,692
Drafts Payable	(9,213)	(30,059)
Cash, Cash Equivalents, and Short-term Investments	529,069	589,375
Payroll Withholdings Included in Cash Balance	273,500	342,000
Client Prepayments Included in Cash Balance	(58,900)	(35,300)
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings	0	0
Short-term Investments	34,292	34,529
Cash	0	0
Drafts Payable	0	0
Cash, Cash Equivalents, and Short-term Investments	34,292	34,529
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings	432,607	503,221
Short-term Investments	48,278	47,992
Cash	23,105	33,692
Drafts Payable	(9,213)	(30,059)
Cash, Cash Equivalents, and Short-term Investments	$ 494,777	$ 554,846