Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Cash and Cash Equivalents [Line Items]
Overnight Holdings	$ 566,030	$ 490,154
Short-term Investments	45,676	120,742
Cash	26,736	47,331
Drafts Payable	(29,178)	(50,624)
Cash, Cash Equivalents, and Short-term Investments	609,264	607,603
Payroll Withholdings Included in Cash Balance	338,300	424,800
Client Prepayments Included in Cash Balance	(117,800)	(20,100)
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings	0	0
Short-term Investments	32,610	31,791
Cash	0	0
Drafts Payable	0	0
Cash, Cash Equivalents, and Short-term Investments	32,610	31,791
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings	566,030	490,154
Short-term Investments	13,066	88,951
Cash	26,736	47,331
Drafts Payable	(29,178)	(50,624)
Cash, Cash Equivalents, and Short-term Investments	$ 576,654	$ 575,812