Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Cash and Cash Equivalents [Line Items]
Overnight Holdings	$ 217,270	$ 311,158
Short-term Investments	77,036	77,492
Cash	24,465	33,207
Drafts Payable	(14,452)	(34,303)
Cash, Cash Equivalents, and Short-term Investments	304,319	387,554
Payroll Withholdings Included in Cash Balance	153,800	224,500
Client Prepayments Included in Cash Balance	19,700	34,200
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings	0	0
Short-term Investments	60,880	60,781
Cash	0	0
Drafts Payable	0	0
Cash, Cash Equivalents, and Short-term Investments	60,880	60,781
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings	217,270	311,158
Short-term Investments	16,156	16,711
Cash	24,465	33,207
Drafts Payable	(14,452)	(34,303)
Cash, Cash Equivalents, and Short-term Investments	$ 243,439	$ 326,773