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Debt (Parenthetical) (Detail) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Apr. 30, 2017
Aug. 31, 2016
Debt Instrument [Line Items]        
Debt instrument, outstanding $ 3,337,100,000      
3.75% Senior Notes due 2021        
Debt Instrument [Line Items]        
Senior notes, issuance amount $ 500,000,000 $ 500,000,000    
Debt Instrument, Interest Rate, Stated Percentage 3.75%      
6 5/8% Senior Notes due 2029        
Debt Instrument [Line Items]        
Senior notes, issuance amount $ 300,000,000 300,000,000    
Debt Instrument, Interest Rate, Stated Percentage 6.625%      
6.25% Senior Notes due 2032        
Debt Instrument [Line Items]        
Senior notes, issuance amount $ 400,000,000 400,000,000    
Debt Instrument, Interest Rate, Stated Percentage 6.25%      
4.35% Senior Notes due 2044        
Debt Instrument [Line Items]        
Senior notes, issuance amount $ 350,000,000 350,000,000    
Debt Instrument, Interest Rate, Stated Percentage 4.35%      
3.70% Senior Notes due 2045        
Debt Instrument [Line Items]        
Senior notes, issuance amount $ 400,000,000 400,000,000    
Debt Instrument, Interest Rate, Stated Percentage 3.70%      
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067        
Debt Instrument [Line Items]        
Senior notes, issuance amount $ 0 1,000,000,000    
Debt instrument, outstanding $ 0 594,600,000    
Debt Instrument, Interest Rate, Stated Percentage 6.70%      
2.45% Senior Notes Due 2027        
Debt Instrument [Line Items]        
Senior notes, issuance amount $ 500,000,000 500,000,000   $ 500,000,000
Debt Instrument, Interest Rate, Stated Percentage 2.45%     2.45%
4.125% Senior Notes due 2047        
Debt Instrument [Line Items]        
Senior notes, issuance amount $ 850,000,000 0 $ 850,000,000  
Debt Instrument, Interest Rate, Stated Percentage 4.125%   4.125%  
Discretionary Line of Credit        
Debt Instrument [Line Items]        
Proceeds from Lines of Credit $ 0 $ 0