Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Overnight holdings:
Money market funds (cash equivalents)	$ 247,720	$ 271,840
Investment holdings:
Money market funds (cash equivalents)	26,048	14,125
Marketable securities	9,875	28,631
Total cash equivalents and marketable securities	283,643	314,596
Cash held in demand accounts	19,377	20,369
Outstanding checks	(23,607)	(29,878)
Total cash, cash equivalents and marketable securities	279,413	305,087
Cash and cash equivalents	269,538	276,456	$ 225,755	$ 264,544
Marketable securities	9,875	28,631
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance	185,700	152,100
Customer prepayments included in cash balance	17,037	87,887
Assets, Fair Value Disclosure [Abstract]
Money market funds	273,768	285,965
Fair Value	9,875	28,631
Total	283,643	314,596
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Amortized Cost Basis	9,875	28,626
Available-for-sale Securities, Gross Unrealized Gain	3	16
Available-for-sale Securities, Gross Unrealized Loss	(3)	(11)
Fair Value	9,875	28,631
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Less than one year	7,341
One to five years	2,534
Amortized cost	9,875
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Less than one year	7,341
One to five years	2,534
Fair Value	9,875	28,631
Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Money market funds	273,768	285,965
Fair Value	0	0
Total	273,768	285,965
Available-for-sale Securities [Abstract]
Fair Value	0	0
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Fair Value	0	0
Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Money market funds	0	0
Fair Value	9,875	28,631
Total	9,875	28,631
Available-for-sale Securities [Abstract]
Fair Value	9,875	28,631
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Fair Value	9,875	28,631
Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Money market funds	0	0
Fair Value	0	0
Total	0	0
Available-for-sale Securities [Abstract]
Fair Value	0	0
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Fair Value	$ 0	$ 0