Cash, Cash Equivalents and Marektable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Cash and Cash Equivalents [Line Items]
Money Market Funds, Overnight Holdings, at Carrying Value	$ 349,857	$ 311,158
Short-term Investments	47,946	77,492
Cash	36,521	33,207
Drafts Payable	(32,254)	(34,303)
Cash, Cash Equivalents, and Short-term Investments	402,070	387,554
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance	234,600	224,500
Client Prepayments Included in Cash Balance	(59,612)	(34,177)
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Money Market Funds, Overnight Holdings, at Carrying Value	0	0
Short-term Investments	34,728	60,781
Cash	0	0
Drafts Payable	0	0
Cash, Cash Equivalents, and Short-term Investments	34,728	60,781
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Money Market Funds, Overnight Holdings, at Carrying Value	349,857	311,158
Short-term Investments	13,218	16,711
Cash	36,521	33,207
Drafts Payable	(32,254)	(34,303)
Cash, Cash Equivalents, and Short-term Investments	$ 367,342	$ 326,773