Cash, Cash Equivalents and Marektable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Overnight holdings:
Money market funds (cash equivalents)	$ 338,112	$ 255,091
Investment holdings:
Money market funds (cash equivalents)	22,634	28,231
Marketable securities	1,960	1,851
Total cash equivalents and marketable securities	362,706	285,173
Cash held in demand accounts	26,700	25,758
Outstanding checks	(33,186)	(23,046)
Total cash, cash equivalents and marketable securities	356,220	287,885
Cash and cash equivalents	354,260	286,034	$ 269,538	$ 276,456
Marketable securities	1,960	1,851
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance	271,500	221,700
Customer prepayments included in cash balance	$ 23,603	$ 21,256