Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Cash and Cash Equivalents [Line Items]
Overnight Holdings	$ 443,760	$ 490,154
Short-term Investments	75,710	120,742
Cash	37,004	47,331
Drafts Payable	(14,824)	(50,624)
Cash, Cash Equivalents, and Short-term Investments	541,650	607,603
Payroll Withholdings Included in Cash Balance	258,200	424,800
Client Prepayments Included in Cash Balance	(116,600)	(20,100)
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings	0	0
Short-term Investments	30,781	31,791
Cash	0	0
Drafts Payable	0	0
Cash, Cash Equivalents, and Short-term Investments	30,781	31,791
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings	443,760	490,154
Short-term Investments	44,929	88,951
Cash	37,004	47,331
Drafts Payable	(14,824)	(50,624)
Cash, Cash Equivalents, and Short-term Investments	$ 510,869	$ 575,812