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Quarterly Financial Information (Detail) (USD $)
In Thousands, except Per Share data, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Quarterly Financial Information [Line Items]      
Operating revenues $ 2,723,414 $ 2,565,943 $ 1,462,931
Operating income 497,361 525,601 200,925
Income from continuing operations, net of income taxes 299,477 322,780 117,898
Loss from discontinued operations, net of income taxes    (367) (956)
Net income 299,477 322,413 116,942
Basic income per common share:      
From continuing operations $ 1.96 $ 2.08 $ 0.77
From discontinued operations $ 0.00 $ 0.00 $ (0.01)
Net income $ 1.96 $ 2.08 $ 0.76
Diluted income per common share:      
From continuing operations $ 1.96 $ 2.06 $ 0.76
From discontinued operations $ 0.00 $ 0.00 $ (0.01)
Net income $ 1.96 $ 2.06 $ 0.76
First Quarter
      
Quarterly Financial Information [Line Items]      
Operating revenues 745,921 567,404  
Operating income 157,664 117,547  
Income from continuing operations, net of income taxes 97,274 71,619  
Loss from discontinued operations, net of income taxes   (367)  
Net income 97,274 71,252  
Basic income per common share:      
From continuing operations $ 0.62 $ 0.46  
From discontinued operations $ 0.00 $ 0.00  
Net income $ 0.62 $ 0.46  
Diluted income per common share:      
From continuing operations $ 0.62 $ 0.46  
From discontinued operations $ 0.00 $ 0.00  
Net income $ 0.62 $ 0.46  
Second Quarter
      
Quarterly Financial Information [Line Items]      
Operating revenues 681,112 600,064  
Operating income 150,894 131,860  
Income from continuing operations, net of income taxes 92,538 81,638  
Net income 92,538 81,638  
Basic income per common share:      
From continuing operations $ 0.60 $ 0.53  
From discontinued operations $ 0.00 $ 0.00  
Net income $ 0.60 $ 0.53  
Diluted income per common share:      
From continuing operations $ 0.60 $ 0.52  
From discontinued operations $ 0.00 $ 0.00  
Net income $ 0.60 $ 0.52  
Third Quarter
      
Quarterly Financial Information [Line Items]      
Operating revenues 643,631 673,828  
Operating income 88,594 132,294  
Income from continuing operations, net of income taxes 50,806 81,928  
Net income 50,806 81,928  
Basic income per common share:      
From continuing operations $ 0.34 $ 0.53  
From discontinued operations $ 0.00 $ 0.00  
Net income $ 0.34 $ 0.53  
Diluted income per common share:      
From continuing operations $ 0.33 $ 0.53  
From discontinued operations $ 0.00 $ 0.00  
Net income $ 0.33 $ 0.53  
Fourth Quarter
      
Quarterly Financial Information [Line Items]      
Operating revenues 652,750 724,647  
Operating income 100,209 143,900  
Income from continuing operations, net of income taxes 58,859 87,595  
Net income $ 58,859 $ 87,595  
Basic income per common share:      
From continuing operations $ 0.40 $ 0.56  
From discontinued operations $ 0.00 $ 0.00  
Net income $ 0.40 $ 0.56  
Diluted income per common share:      
From continuing operations $ 0.40 $ 0.56  
From discontinued operations $ 0.00 $ 0.00  
Net income $ 0.40 $ 0.56