Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Overnight holdings
Money market funds (cash equivalents)	$ 305,198	$ 338,112
Investment holdings
Money market funds (cash equivalents)	22,618	22,634
Marketable securities	1,928	1,960
Total cash equivalents and marketable securities	329,744	362,706
Cash held in demand accounts	41,814	26,700
Outstanding checks	(14,989)	(33,186)
Total cash, cash equivalents and marketable securities	356,569	356,220
Cash and cash equivalents	354,641	354,260
Marketable securities	1,928	1,960
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance	254,200	271,500
Client prepayments included in cash balance	$ 14,900	$ 23,600