Debt (Details) (USD $)	3 Months Ended			3 Months Ended	6 Months Ended
	Jun. 30, 2011 Convertible Senior Subordinated Notes [Member]	Dec. 31, 2010 Convertible Senior Subordinated Notes [Member]	Dec. 13, 2006 Convertible Senior Subordinated Notes [Member]	Jun. 30, 2011 Credit Facility [Member]	Jun. 30, 2011 Credit Facility [Member]	Dec. 31, 2007 Prior Credit Facility [Member]
Debt Instrument [Line Items]
Face amount			$ 143,800,000			$ 100,000,000
Interest rate (in hundredths)			2.4375%
Maturity date	2027
Net carrying amount	128,300,000	125,500,000
Unamortized debt discount	15,500,000	18,300,000
Initial conversion rate	40.8513
Dollars of debt per conversion rate	1,000
Initial conversion price	$ 24.48
Fair value of debt	149,000,000	150,200,000
Maximum borrowing capacity				300,000,000	300,000,000
Expiration date of credit facility				June 7, 2016
Capacity for the issuance of standby letters of credit				125,000,000	125,000,000
Maximum increase in borrowing capacity				150,000,000	150,000,000
Variable rate basis of loans under the credit facility				LIBOR	LIBOR
Basis spread on variable rate, minimum (in hundredths)				1.75%	1.75%
Basis spread on variable rate, maximum (in hundredths)				2.50%	2.50%
Basis spread on prime base rate, at the company's election, minimum (in hundredths)				0.75%	0.75%
Basis spread on prime base rate, at the company's election, maximum (in hundredths)				1.50%	1.50%
Commitment fee for the unused portion of the credit facility, minimum (in hundredths)				0.30%
Commitment fee for the unused portion of the credit facility, maximum (in hundredths)				0.50%
Letters of credit issued				17,700,000	17,700,000
Amount available for borrowings				$ 282,300,000	$ 282,300,000