Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Earnings Per Share [Abstract]
Net income for basic and diluted earnings per share	$ 7,018	$ 365,701	$ 24,747	$ 76,226	$ (392,803)	$ 367,779	$ (3,653)	$ 46,964	$ 473,692	$ 18,287	$ 607,421
Weighted-average common shares outstanding (in shares)									219,638	217,930	216,983
Dilutive effect of stock options, performance stock awards and restricted stock awards (in shares)									1,303	1,527	2,246
Diluted weighted-average shares outstanding									220,941	219,457	219,229
Basic earnings per share (in dollars per share)	$ 0.03	$ 1.66	$ 0.11	$ 0.35	$ (1.80)	$ 1.69	$ (0.02)	$ 0.22	$ 2.16	$ 0.08	$ 2.80
Diluted earnings per share (in dollars per share)	$ 0.03	$ 1.65	$ 0.11	$ 0.35	$ (1.80)	$ 1.68	$ (0.02)	$ 0.21	$ 2.14	$ 0.08	$ 2.77