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Debt (Details) - Debt Issuance - USD ($)
$ in Millions
12 Months Ended
Mar. 04, 2019
Mar. 07, 2018
May 22, 2017
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Debt Instrument            
Net proceeds of issuance, after the deduction of the underwriting discount and expenses       $ 492 $ 591 $ 789
Senior Notes [Member] | 4.10% Senior notes due March 4, 2049 [Member]            
Debt Instrument            
Debt, principal amount $ 500          
Interest rate (percent) 4.10%     4.10%    
Net proceeds of issuance, after the deduction of the underwriting discount and expenses $ 492          
Percentage of principal amount at which redemption price may be set 100.00%          
Basis points added to current Treasury rate used in calculation of alternative redemption price 0.20%          
Senior Notes [Member] | 4.05% Senior notes due March 7, 2048 [Member]            
Debt Instrument            
Debt, principal amount   $ 500        
Interest rate (percent)   4.05%   4.05% 4.05%  
Net proceeds of issuance, after the deduction of the underwriting discount and expenses   $ 491        
Percentage of principal amount at which redemption price may be set   100.00%        
Basis points added to current Treasury rate used in calculation of alternative redemption price   0.15%        
Senior Notes [Member] | 4.00% Senior notes due May 30, 2047 [Member]            
Debt Instrument            
Debt, principal amount     $ 700      
Interest rate (percent)     4.00% 4.00% 4.00%  
Net proceeds of issuance, after the deduction of the underwriting discount and expenses     $ 689      
Percentage of principal amount at which redemption price may be set     100.00%      
Basis points added to current Treasury rate used in calculation of alternative redemption price     0.15%