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Quarterly Financial Information (Detail) (USD $)
In Thousands, except Per Share data, unless otherwise specified
 12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Quarterly Financial Information [Line Items]      
Operating revenues $ 2,565,943 $ 1,462,931 $ 781,946
Operating income 525,601 200,925 (48,214)
Income from continuing operations, net of income taxes 322,780 117,898 (33,960)
Loss from discontinued operations, net of income taxes (367) (956) (4,330)
Net income 322,413 116,942 (38,290)
Basic income (loss) per common share:      
From continuing operations $ 2.08 $ 0.77 $ (0.22)
From discontinued operations $ 0.00 $ (0.01) $ (0.03)
Net income $ 2.08 $ 0.76 $ (0.25)
Diluted income (loss) per common share:      
From continuing operations $ 2.06 $ 0.76 $ (0.22)
From discontinued operations $ 0.00 $ (0.01) $ (0.03)
Net income $ 2.06 $ 0.76 $ (0.25)
First Quarter
      
Quarterly Financial Information [Line Items]      
Operating revenues 567,404 271,598  
Operating income 117,547 7,831  
Income from continuing operations, net of income taxes 71,619 4,186  
Loss from discontinued operations, net of income taxes (367)    
Net income 71,252 4,186  
Basic income (loss) per common share:      
From continuing operations $ 0.46 $ 0.03  
From discontinued operations $ 0.00 $ 0.00  
Net income $ 0.46 $ 0.03  
Diluted income (loss) per common share:      
From continuing operations $ 0.46 $ 0.03  
From discontinued operations $ 0.00 $ 0.00  
Net income $ 0.46 $ 0.03  
Second Quarter
      
Quarterly Financial Information [Line Items]      
Operating revenues 600,064 306,992  
Operating income 131,860 45,757  
Income from continuing operations, net of income taxes 81,638 29,528  
Net income 81,638 29,528  
Basic income (loss) per common share:      
From continuing operations $ 0.53 $ 0.19  
From discontinued operations $ 0.00 $ 0.00  
Net income $ 0.53 $ 0.19  
Diluted income (loss) per common share:      
From continuing operations $ 0.52 $ 0.19  
From discontinued operations $ 0.00 $ 0.00  
Net income $ 0.52 $ 0.19  
Third Quarter
      
Quarterly Financial Information [Line Items]      
Operating revenues 673,828 378,663  
Operating income 132,294 52,509  
Income from continuing operations, net of income taxes 81,928 29,374  
Net income 81,928 29,374  
Basic income (loss) per common share:      
From continuing operations $ 0.53 $ 0.19  
From discontinued operations $ 0.00 $ 0.00  
Net income $ 0.53 $ 0.19  
Diluted income (loss) per common share:      
From continuing operations $ 0.53 $ 0.19  
From discontinued operations $ 0.00 $ 0.00  
Net income $ 0.53 $ 0.19  
Fourth Quarter
      
Quarterly Financial Information [Line Items]      
Operating revenues 724,647 505,678  
Operating income 143,900 94,828  
Income from continuing operations, net of income taxes 87,595 54,810  
Loss from discontinued operations, net of income taxes   (956)  
Net income $ 87,595 $ 53,854  
Basic income (loss) per common share:      
From continuing operations $ 0.56 $ 0.36  
From discontinued operations $ 0.00 $ (0.01)  
Net income $ 0.56 $ 0.35  
Diluted income (loss) per common share:      
From continuing operations $ 0.56 $ 0.35  
From discontinued operations $ 0.00 $ (0.01)  
Net income $ 0.56 $ 0.35