Earnings per Share (Details) (USD $)	3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Performance Share Awards Under the Equity Incentive Compensation Plan	Sep. 30, 2012 Performance Share Awards Under the Equity Incentive Compensation Plan	Sep. 30, 2012 Convertible Debt [Member]	Sep. 30, 2012 Convertible Debt [Member]	Mar. 31, 2012 Convertible Debt [Member]	Dec. 31, 2011 Convertible Debt [Member]
Earnings per share
Performance period to receive awards (in years)			3 years			3
Common stock awards vesting multiplier, low end of range					0	0
Common stock awards vesting multiplier, high end of range					2	2
Interest rate on notes (as a percent)							3.50%	3.50%		3.50%
Stock Issued During Period, Shares, Conversion of Convertible Securities								864,106
Debt Instrument, Principal Amount Denominator for Conversion into Common Stock							$ 1,000
Common stock conversion price (in dollars per share)									$ 54.42
Calculation of basic and diluted earnings per share
Net income	$ (38,336,000)	$ 230,097,000	$ 12,889,000	$ 336,127,000
Basic weighted-average common shares outstanding (in shares)	65,745,000	63,904,000	64,815,000	63,665,000
Add: dilutive effect of stock options, unvested RSUs, and contingent PSAs (in shares)	0	2,062,000	1,870,000	2,589,000
Add: dilutive effect of 3.50% Senior Convertible Notes (in shares)	0	1,420,000	658,000	1,136,000
Diluted weighted-average common shares outstanding (in shares)	65,745,000	67,386,000	67,343,000	67,390,000
Basic net income per common share (in dollars per share)	$ (0.58)	$ 3.60	$ 0.20	$ 5.28
Diluted net income per common share (in dollars per share)	$ (0.58)	$ 3.41	$ 0.19	$ 4.99