Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Overnight holdings
Money market funds (cash equivalents)	$ 282,337	$ 338,112
Investment holdings
Money market funds (cash equivalents)	17,354	22,634
Marketable securities	7,207	1,960
Total cash equivalents and marketable securities	306,898	362,706
Cash held in demand accounts	25,053	26,700
Outstanding checks	(16,033)	(33,186)
Total cash, cash equivalents and marketable securities	315,918	356,220
Cash and cash equivalents	308,711	354,260	$ 241,890	$ 286,034
Marketable securities	7,207	1,960
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance	191,900	271,500
Client prepayments included in cash balance	$ 14,000	$ 23,600