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Debt (Details) - USD ($)
9 Months Ended
Mar. 07, 2018
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Debt Instrument        
Net proceeds of issuance, after the deduction of the underwriting discount and expenses   $ 591,000,000 $ 689,000,000  
Commercial paper outstanding   $ 100,000,000   $ 100,000,000
Senior Notes [Member] | 4.05% Senior notes due March 7, 2048 [Member]        
Debt Instrument        
Debt, principal amount $ 500,000,000      
Interest rate (percent) 4.05%      
Net proceeds of issuance, after the deduction of the underwriting discount and expenses $ 491,000,000      
Percentage of principal amount at which redemption price may be set 100.00% 100.00%    
Basis points added to current Treasury rate used in calculation of alternative redemption price 0.15%