Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Overnight holdings
Money market funds (cash equivalents)	$ 201,660	$ 255,091
Investment holdings
Money market funds (cash equivalents)	28,567	28,231
Marketable securities	1,929	1,851
Total cash equivalents and marketable securities	232,156	285,173
Cash held in demand accounts	27,586	25,758
Outstanding checks	(15,923)	(23,046)
Total cash, cash equivalents and marketable securities	243,819	287,885
Cash and cash equivalents	241,890	286,034	$ 271,573	$ 269,538
Marketable securities	1,929	1,851
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance	173,300	221,700
Client prepayments included in cash balance	$ 22,300	$ 21,300