Convertible Senior Notes due 2016 (Details) (Convertible senior notes due 2016, USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 item	Jun. 30, 2012	Dec. 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		$ 230,000,000	$ 230,000,000
Rate of convertible senior notes due 2016 (as a percent)	4.00%		4.00%			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		$ 39.65
Redemption price, percentage of principal	100.00%		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%		130.00%
Carrying amount of the equity component	89,468,000		89,468,000
Net carrying amount of the liability component	91,232,000		91,232,000
Unamortized discount of the liability component	49,295,000		49,295,000
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	4,600,000	4,600,000
Non-cash interest expense relating to amortization of the debt discount	3,200,000	2,800,000	6,300,000	5,600,000
Estimated fair value of senior notes	520,500,000		520,500,000
Number of notes submitted and settled for conversion			5
Face value of notes submitted and settled for conversion	$ 5,000		$ 5,000