Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Cash and Cash Equivalents [Line Items]
Overnight Holdings	$ 314,952	$ 349,857
Short-term Investments	72,861	47,946
Cash	28,259	36,521
Drafts Payable	(9,722)	(32,254)
Cash, Cash Equivalents, and Short-term Investments	406,350	402,070
Payroll Withholdings Included in Cash Balance	147,100	234,600
Client Prepayments Included in Cash Balance	(46,500)	(59,600)
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings	0	0
Short-term Investments	33,994	34,728
Cash	0	0
Drafts Payable	0	0
Cash, Cash Equivalents, and Short-term Investments	33,994	34,728
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings	314,952	349,857
Short-term Investments	38,867	13,218
Cash	28,259	36,521
Drafts Payable	(9,722)	(32,254)
Cash, Cash Equivalents, and Short-term Investments	$ 372,356	$ 367,342