Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Cash and Cash Equivalents [Line Items]
Overnight Holdings	$ 404,564	$ 503,221
Short-term Investments	82,610	82,521
Cash	30,377	33,692
Drafts Payable	(16,933)	(30,059)
Cash, Cash Equivalents, and Short-term Investments	500,618	589,375
Payroll Withholdings Included in Cash Balance	198,400	342,000
Client Prepayments Included in Cash Balance	(74,600)	(35,300)
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings	0	0
Short-term Investments	32,697	34,529
Cash	0	0
Drafts Payable	0	0
Cash, Cash Equivalents, and Short-term Investments	32,697	34,529
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings	404,564	503,221
Short-term Investments	49,913	47,992
Cash	30,377	33,692
Drafts Payable	(16,933)	(30,059)
Cash, Cash Equivalents, and Short-term Investments	$ 467,921	$ 554,846