Convertible Senior Notes due 2016 (Details) (Convertible senior notes due 2016, USD $)	3 Months Ended
	Mar. 31, 2013 item	Mar. 31, 2012	Aug. 31, 2009
Convertible senior notes due 2016
Debt disclosures
Aggregate principal amount of senior notes due 2016	$ 230,000,000		$ 230,000,000
Rate of convertible senior notes due 2016 (as a percent)			4.00%
Conversion rate (shares of common stock per $1,000 principal amount of debentures)	25.2207
Principal amount per debenture used in conversion rate	1,000
Conversion price (per share of common stock)	$ 39.65
Redemption price, percentage of principal	100.00%
Minimum number of trading days for reported sale price of the common stock	20 days
Number of trading days prior to notice of redemption	30 days
Redemption provision, stock price as percentage of conversion price	130.00%
Carrying amount of the equity component	89,468,000
Net carrying amount of the liability component	88,039,000
Unamortized discount of the liability component	52,492,000
Effective interest rate (as a percent)	12.50%
Equity component of convertible debt	89,500,000
Cash interest expense on debt	2,300,000	2,300,000
Non-cash interest expense relating to amortization of the debt discount	3,100,000	2,700,000
Estimated fair value of senior notes	537,000,000
Number of notes submitted and settled for conversion	5
Face value of notes submitted and settled for conversion	$ 5,000