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Debt (Details) - USD ($)
3 Months Ended
Mar. 04, 2019
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Debt Instrument        
Net proceeds of issuance, after the deduction of the underwriting discount and expenses   $ 492,000,000 $ 491,000,000  
Commercial paper outstanding   $ 100,000,000   $ 100,000,000
Senior Notes [Member] | 4.10% Senior notes due March 4, 2049 [Member]        
Debt Instrument        
Debt, principal amount $ 500,000,000      
Interest rate (percent) 4.10%      
Net proceeds of issuance, after the deduction of the underwriting discount and expenses $ 492,000,000      
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%