Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Cash and Cash Equivalents [Line Items]
Overnight Holdings	$ 387,027	$ 503,221
Short-term Investments	82,584	82,521
Cash	31,931	33,692
Drafts Payable	(11,558)	(30,059)
Cash, Cash Equivalents, and Short-term Investments	489,984	589,375
Payroll Withholdings Included in Cash Balance	226,700	342,000
Client Prepayments Included in Cash Balance	(50,400)	(35,300)
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings	0	0
Short-term Investments	34,597	34,529
Cash	0	0
Drafts Payable	0	0
Cash, Cash Equivalents, and Short-term Investments	34,597	34,529
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings	387,027	503,221
Short-term Investments	47,987	47,992
Cash	31,931	33,692
Drafts Payable	(11,558)	(30,059)
Cash, Cash Equivalents, and Short-term Investments	$ 455,387	$ 554,846