Description of the Business and Summary of Significant Accounting Policies (Details 5) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net Income (Numerator)
Basic earnings - net income (loss) (in dollars)	$ 60,571	$ 12,833	$ 44,472	$ 45,675	$ 30,203	$ 43,304	$ 98,710	$ 105,903	$ 163,551	$ 278,120	$ 140,709
5.125% convertible senior notes, net of tax (in dollars)									9,432	9,432	9,508
Diluted earnings - net income (loss) (in dollars)									$ 172,983	$ 287,552	$ 150,217
Shares (Denominator)
Weighted average common shares outstanding (in shares)									219,159,000	218,471,000	216,760,000
Dilutive stock options, deferred stock units, and restricted stock units (in shares)									916,000	1,139,000	1,575,000
5.125% convertible senior notes, net of tax (in shares)									16,549,000	16,382,000	16,382,000
Weighted average common shares outstanding - diluted (in shares)									236,624,000	235,992,000	234,717,000
Per Share Amount
Basic earnings per share (in dollars per share)	$ 0.28	$ 0.06	$ 0.20	$ 0.21	$ 0.14	$ 0.20	$ 0.45	$ 0.49	$ 0.75	$ 1.27	$ 0.65
Diluted earnings per share (in dollars per share)	$ 0.27	$ 0.06	$ 0.20	$ 0.20	$ 0.14	$ 0.19	$ 0.43	$ 0.46	$ 0.73	$ 1.22	$ 0.64
5.125% convertible senior notes, due 2014
Antidilutive securities excluded from computation of earnings per share amount
Stated interest rate on convertible senior notes (as a percent)	5.125%				5.125%				5.125%	5.125%
Options
Antidilutive securities excluded from computation of earnings per share amount
Antidilutive securities excluded from computation of earnings per share amount (in shares)									4,500,000	4,000,000	3,100,000