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Quarterly Financial Information (Detail) (USD $)
In Thousands, except Per Share data, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Quarterly Financial Information [Line Items]      
Operating revenues $ 2,716,034 $ 2,723,414 $ 2,565,943
Operating income 322,191 497,361 525,601
Net income 188,009 299,477 322,413
Net income per common share:      
Basic $ 1.29 $ 1.96 $ 2.08
Diluted $ 1.28 $ 1.96 $ 2.06
First Quarter
      
Quarterly Financial Information [Line Items]      
Operating revenues 667,039 745,921  
Operating income 94,932 157,664  
Net income 56,230 97,274  
Net income per common share:      
Basic $ 0.38 $ 0.62  
Diluted $ 0.38 $ 0.62  
Second Quarter
      
Quarterly Financial Information [Line Items]      
Operating revenues 659,316 681,112  
Operating income 70,606 150,894  
Net income 40,768 92,538  
Net income per common share:      
Basic $ 0.28 $ 0.60  
Diluted $ 0.28 $ 0.60  
Third Quarter
      
Quarterly Financial Information [Line Items]      
Operating revenues 730,907 643,631  
Operating income 124,551 88,594  
Net income 74,420 50,806  
Net income per common share:      
Basic $ 0.51 $ 0.34  
Diluted $ 0.51 $ 0.33  
Fourth Quarter
      
Quarterly Financial Information [Line Items]      
Operating revenues 658,772 652,750  
Operating income 32,102 100,209  
Net income $ 16,591 $ 58,859  
Net income per common share:      
Basic $ 0.12 $ 0.40  
Diluted $ 0.11 $ 0.40