Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Cash and Cash Equivalents [Line Items]
Overnight Holdings	$ 398,668	$ 349,857
Short-term Investments	75,230	47,946
Cash	29,378	36,521
Drafts Payable	(11,925)	(32,254)
Cash, Cash Equivalents, and Short-term Investments	491,351	402,070
Payroll Withholdings Included in Cash Balance	190,900	234,600
Client Prepayments Included in Cash Balance	(31,500)	(59,600)
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings	0	0
Short-term Investments	31,952	34,728
Cash	0	0
Drafts Payable	0	0
Cash, Cash Equivalents, and Short-term Investments	31,952	34,728
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings	398,668	349,857
Short-term Investments	43,278	13,218
Cash	29,378	36,521
Drafts Payable	(11,925)	(32,254)
Cash, Cash Equivalents, and Short-term Investments	$ 459,399	$ 367,342