Cash, Cash Equivalents and Marektable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Line Items]
Money Market Funds, Overnight Holdings, at Carrying Value	$ 311,158	$ 338,112
Short-term Investments	77,492	24,594
Cash	33,207	26,700
Drafts Payable	(34,303)	(33,186)
Cash, Cash Equivalents, and Short-term Investments	387,554	356,220
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance	224,500	271,500
Client Prepayments Included in Cash Balance	34,177	23,603
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Money Market Funds, Overnight Holdings, at Carrying Value	0	0
Short-term Investments	60,781	1,960
Cash	0	0
Drafts Payable	0	0
Cash, Cash Equivalents, and Short-term Investments	60,781	1,960
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Money Market Funds, Overnight Holdings, at Carrying Value	311,158	338,112
Short-term Investments	16,711	22,634
Cash	33,207	26,700
Drafts Payable	(34,303)	(33,186)
Cash, Cash Equivalents, and Short-term Investments	$ 326,773	$ 354,260