Quarterly Financial Data (Unaudited) (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 Data (Unaudited)
Operating revenues	$ 899,850	$ 955,425	$ 933,697	$ 865,951	$ 796,285	$ 686,858	$ 649,759	$ 632,243	$ 3,654,923	$ 2,765,145	$ 2,613,614
Operating income (loss)	(4,988)	26,827	19,040	227	51,835	58,282	49,288	42,421	41,105	201,826	212,195
Net income (loss)	$ (17,967)	$ 116	$ 1,579	$ (11,063)	$ 37,207	$ 25,474	$ 18,655	$ 15,014	$ (27,335)	$ 96,350	$ 83,658
Net income (loss) per common share:
Basic (in dollars per share)	$ (0.35)	$ 0	$ 0.03	$ (0.21)	$ 0.68	$ 0.46	$ 0.33	$ 0.27	$ (0.52)	$ 1.73	$ 1.50
Diluted (in dollars per share)	$ (0.35)	$ 0	$ 0.03	$ (0.21)	$ 0.67	$ 0.45	$ 0.33	$ 0.26	$ (0.52)	$ 1.70	$ 1.47
Weighted average common shares:
Basic (in shares)	50,691	51,570	52,698	53,844	54,747	55,901	55,936	55,855	52,201	55,610	55,854
Diluted (in shares)	50,691	52,315	53,371	53,844	55,719	56,804	56,718	56,864	52,201	56,526	56,814