Debt Obligations (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Debt issuance costs	$ 1,500,000
Debt issuance cost amortization period	5 years
Revolving loans	300,000,000
Capacity for the issuance of standby letters of credit	125,000,000
Maximum increase in borrowing capacity	150,000,000
Letters of credit issued	4,000,000
Amount available for borrowings	250,000,000
Debt extinguishment expense	0	10,261,000	0
Long-term Debt	143,236,000	150,736,000
Current portion of long-term obligations	(8,236,000)	(7,500,000)
Long-term Debt, Excluding Current Maturities	135,000,000	143,236,000
Convertible Senior Subordinated Notes [Member]
Debt Instrument [Line Items]
Interest rate (in hundredths)	2.4375%
Maturity date	Dec. 31, 2027
Accelerated amortization of debt discount		6,800,000
Prepayment premiums and other expenses		2,800,000
Accelerated amortization of debt issuance costs		700,000
Long-term Debt	736,000	736,000
Credit Facility [Member]
Debt Instrument [Line Items]
Maturity date	Dec. 12, 2017
Variable rate basis of loans under the credit facility	LIBOR
Basis spread on base rate, minimum (in hundredths)	0.75%
Basis spread on base rate, maximum (in hundredths)	1.50%
Credit Facility [Member] | Minimum [Member]
Debt Instrument [Line Items]
Basis spread on variable rate (in hundredths)	1.75%
Commitment fee for the unused portion of the credit facility (in hundredths)	0.30%
Credit Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Basis spread on variable rate (in hundredths)	2.50%
Commitment fee for the unused portion of the credit facility (in hundredths)	0.50%
Senior Secured Term Loan [Member]
Debt Instrument [Line Items]
Fair value of debt	142,500,000	150,000,000	0
Face amount	150,000,000
Maturity date	Dec. 12, 2017
Variable rate basis of loans under the credit facility	LIBOR
Basis spread on base rate, minimum (in hundredths)	0.75%
Quarterly principal payment amount	1,900,000
Remaining principal amount	114,400,000
Basis spread on variable rate (in hundredths)	1.75%
Long-term Debt	$ 142,500,000	$ 150,000,000