Cash, Cash Equivalents and Marektable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Overnight holdings:
Money market funds (cash equivalents)	$ 255,091	$ 247,720
Investment holdings:
Money market funds (cash equivalents)	28,231	26,048
Marketable securities	1,851	9,875
Total cash equivalents and marketable securities	285,173	283,643
Cash held in demand accounts	25,758	19,377
Outstanding checks	(23,046)	(23,607)
Total cash, cash equivalents and marketable securities	287,885	279,413
Cash and cash equivalents	286,034	269,538	$ 276,456	$ 225,755
Marketable securities	1,851	9,875
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance	221,700	185,700
Customer prepayments included in cash balance	$ 21,256	$ 17,037