Earnings Per Share (Details) (USD $) In Thousands, except Per 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
Earnings Per Share
Net income for basic and diluted earnings per share (in dollars)	$ (392,803)	$ 367,779	$ (3,653)	$ 46,964	$ 36,562	$ 398,958	$ 93,491	$ 78,410	$ 18,287	$ 607,421	$ 515,653
Weighted-average common shares outstanding									217,930	216,983	215,026
Dilutive effect of stock options, performance stock awards and restricted stock awards (in shares)									1,527	2,246	2,685
Diluted weighted-average shares outstanding									219,457	219,229	217,711
Basic earnings per share (in dollars per share)	$ (1.80)	$ 1.69	$ (0.02)	$ 0.22	$ 0.17	$ 1.84	$ 0.43	$ 0.36	$ 0.08	$ 2.80	$ 2.40
Diluted earnings per share (in dollars per share)	$ (1.80)	$ 1.68	$ (0.02)	$ 0.21	$ 0.17	$ 1.82	$ 0.43	$ 0.36	$ 0.08	$ 2.77	$ 2.37