Earnings Per Share (Details) (USD $)	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 5.125% convertible senior notes, due 2014	Dec. 31, 2011 5.125% convertible senior notes, due 2014	Sep. 30, 2012 Stock options	Sep. 30, 2011 Stock options
Antidilutive securities excluded from computation of earnings per share amount
Stated interest rate (as a percent)					5.125%	5.125%
Antidilutive securities excluded from computation of earnings per share amount ( in shares)					16,600,000		6,500,000	5,800,000
After-tax equivalent of interest excluded from the computation of earnings per share (in dollars)					$ 2,400,000
Net Income (Numerator)
Basic earnings (loss) - net income (loss) (in dollars)	12,833,000	43,304,000	102,980,000	247,917,000
5.125% convertible senior notes, net of tax (in dollars)		2,358,000	7,074,000	7,074,000
Diluted earnings (loss) - net income (loss) (in dollars)	$ 12,833,000	$ 45,662,000	$ 110,054,000	$ 254,991,000
Shares (Denominator)
Weighted average common shares outstanding (in shares)	219,191,000	218,674,000	219,097,000	218,389,000
Dilutive stock option effect (in shares)	853,000	703,000	908,000	1,312,000
5.125% convertible senior notes, net of tax (in shares)		16,382,000	16,531,000	16,382,000
Weighted average common shares outstanding - diluted (in shares)	220,044,000	235,759,000	236,536,000	236,083,000
Per Share Amount
Basic earnings (loss) per share (in dollars per share)	$ 0.06	$ 0.20	$ 0.47	$ 1.14
Diluted earnings (loss) per share (in dollars per share)	$ 0.06	$ 0.19	$ 0.47	$ 1.08