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Quarterly Financial Data (Unaudited) (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
Quarterly Financial Data (Unaudited)                      
Operating revenues $ 810,726 $ 865,259 $ 937,214 $ 921,173 $ 899,850 $ 955,425 $ 933,697 $ 865,951 $ 3,534,372 $ 3,654,923 $ 2,765,145
Operating income 43,750 54,974 46,806 20,457 (4,989) 26,827 19,040 227 165,987 41,105 201,826
Net income (loss) $ 13,946 $ 20,933 $ 16,960 $ (682) $ (17,967) $ 116 $ 1,579 $ (11,063) $ 51,157 $ (27,335) $ 96,350
Net income (loss) per common share:                      
Basic (in dollars per share) $ 0.27 $ 0.41 $ 0.33 $ (0.01) $ (0.35) $ 0.00 $ 0.03 $ (0.21) $ 1.00 $ (0.52) $ 1.73
Diluted (in dollars per share) $ 0.27 $ 0.40 $ 0.33 $ (0.01) $ (0.35) $ 0.00 $ 0.03 $ (0.21) $ 0.99 $ (0.52) $ 1.70
Weighted average common shares:                      
Basic (in shares) 51,296 51,241 50,944 50,881 50,691 51,570 52,698 53,844 51,090 52,201 55,610
Diluted (in shares) 52,161 52,153 51,789 50,881 50,691 52,315 53,371 53,844 51,746 52,201 56,526