Cash, Cash Equivalents and Marketable Securities (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Available-for-sale Securities [Abstract]
Amortized Cost	$ 28,626,000	$ 46,290,000
Gross Unrealized Gains	16,000	51,000
Gross Unrealized Losses	(11,000)	(1,000)
Fair Value	28,631,000	46,340,000
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Less than one year	12,728,000
One to five years	15,898,000
Amortized cost	28,626,000
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Less than one year	12,737,000
One to five years	15,894,000
Fair Value	28,631,000	46,340,000
Overnight holdings:
Money market funds (cash equivalents)	271,840,000	192,040,000
Investment holdings:
Money market funds (cash equivalents)	14,125,000	42,913,000
Marketable securities	28,631,000	46,340,000
Total cash equivalents and marketable securities	314,596,000	281,293,000
Cash held in demand accounts	20,369,000	23,054,000
Outstanding checks	(29,878,000)	(32,252,000)
Total cash, cash equivalents and marketable securities	305,087,000	272,095,000
Cash and cash equivalents	276,456,000	225,755,000	264,544,000	211,208,000
Marketable securities	28,631,000	46,340,000
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance	152,100,000	143,000,000
Customer prepayments included in cash balance	87,887,000	24,459,000
Assets, Fair Value Disclosure [Abstract]
Money market funds	285,965,000	234,953,000
Fair Value	28,631,000	46,340,000
Total	314,596,000	281,293,000
Level 1 [Member]
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
Assets, Fair Value Disclosure [Abstract]
Money market funds	285,965,000	234,953,000
Fair Value	0	0
Total	285,965,000	234,953,000
Level 2 [Member]
Available-for-sale Securities [Abstract]
Fair Value	28,631,000	46,340,000
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Fair Value	28,631,000	46,340,000
Assets, Fair Value Disclosure [Abstract]
Money market funds	0	0
Fair Value	28,631,000	46,340,000
Total	28,631,000	46,340,000
Level 3 [Member]
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
Assets, Fair Value Disclosure [Abstract]
Money market funds	0	0
Fair Value	0	0
Total	$ 0	$ 0