Cash, Cash Equivalents and Marektable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Cash and Cash Equivalents [Line Items]
Money Market Funds, Overnight Holdings, at Carrying Value	$ 490,154	$ 503,221
Short-term Investments	120,742	82,521
Cash	47,331	33,692
Drafts Payable	(50,624)	(30,059)
Cash, Cash Equivalents, and Short-term Investments	607,603	589,375
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance	424,800	342,000
Client Prepayments Included in Cash Balance	20,054	35,328
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Money Market Funds, Overnight Holdings, at Carrying Value	0	0
Short-term Investments	31,791	34,529
Cash	0	0
Drafts Payable	0	0
Cash, Cash Equivalents, and Short-term Investments	31,791	34,529
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Money Market Funds, Overnight Holdings, at Carrying Value	490,154	503,221
Short-term Investments	88,951	47,992
Cash	47,331	33,692
Drafts Payable	(50,624)	(30,059)
Cash, Cash Equivalents, and Short-term Investments	$ 575,812	$ 554,846