Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Line Items]
Overnight Holdings	$ 270,172	$ 338,112
Short-term Investments	81,565	24,594
Cash	29,585	26,700
Drafts Payable	(15,447)	(33,186)
Cash, Cash Equivalents, and Short-term Investments	365,875	356,220
Payroll Withholdings Included in Cash Balance	181,300	271,500
Client Prepayments Included in Cash Balance	18,000	23,600
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings	0	0
Short-term Investments	37,576	1,960
Cash	0	0
Drafts Payable	0	0
Cash, Cash Equivalents, and Short-term Investments	37,576	1,960
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings	270,172	338,112
Short-term Investments	43,989	22,634
Cash	29,585	26,700
Drafts Payable	(15,447)	(33,186)
Cash, Cash Equivalents, and Short-term Investments	$ 328,299	$ 354,260