Quarterly Financial Information (details) (USD $) In Thousands, except Per 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
Quarterly Financial Information Disclosure [Abstract]
Gross operating revenues	$ 271,198	$ 93,313	$ 161,197	$ 323,088	$ 268,145	$ 95,067	$ 162,365	$ 286,529	$ 848,796	$ 812,106	$ 1,012,711
Net operating revenues	119,910	47,783	67,232	134,508	118,251	46,211	72,193	130,926	369,433	367,581	376,887
Net income	$ 29,244	$ (8,312)	$ 2,193	$ 40,773	$ 29,591	$ (7,420)	$ 6,888	$ 43,608	$ 63,898	$ 72,667	$ 75,122
Earnings Per Share, Basic	$ 1.09	$ (0.31)	$ 0.08	$ 1.53	$ 1.11	$ (0.28)	$ 0.26	$ 1.64	$ 2.39	$ 2.73	$ 2.83
Diluted eps	$ 1.09	$ (0.31)	$ 0.08	$ 1.53	$ 1.11	$ (0.28)	$ 0.26	$ 1.64	$ 2.39	$ 2.73	$ 2.83