Reconciliation of Earnings Per Share (Reconciliation) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended									12 Months Ended
	Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Earnings Per Share Reconciliation
Net Income, Basic EPS	$ 14,432	[1]	$ 87,599	$ 63,325	$ 94,268	$ 80,250	$ 38,304	$ 55,543	$ 116,710	$ 259,624	$ 290,807	$ 256,325
Basic Shares Oustanding (in shares)										72,055,661	71,845,463	71,667,304
Earnings Per Share, Basic (dollars per share)	$ 0.20	[1]	$ 1.22	$ 0.88	$ 1.31	$ 1.12	$ 0.53	$ 0.77	$ 1.63	$ 3.60	$ 4.05	$ 3.58
Effect of dilutive securities
Net Income, Diluted EPS	$ 14,432	[1]	$ 87,599	$ 63,325	$ 94,268	$ 80,250	$ 38,304	$ 55,543	$ 116,710	$ 259,624	$ 290,807	$ 256,325
Diluted Shares Outstanding (in shares)										72,332,369	72,050,997	71,885,422
Earnings Per Share, Diluted (dollars per share)	$ 0.20	[1]	$ 1.21	$ 0.87	$ 1.30	$ 1.11	$ 0.53	$ 0.77	$ 1.62	$ 3.59	$ 4.04	$ 3.57
Performance Share Awards
Effect of dilutive securities
Incremental Common Shares Attributable to Share-based Payment Arrangements										0	0	108,000
Stock options
Effect of dilutive securities
Incremental Common Shares Attributable to Share-based Payment Arrangements										270,000	190,000	78,000
Non-vested restricted stock
Effect of dilutive securities
Incremental Common Shares Attributable to Share-based Payment Arrangements										6,000	16,000	32,000

[1] The three months ended December 31, 2011 includes an after-tax non-cash mark-to-market loss on derivatives of $56.6 million, or $0.78 per diluted share.