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Long-Term Debt (Senior Notes Due 2026 and 2046) (Details) - Senior Notes [Member] - USD ($)
$ in Millions
Apr. 24, 2017
Dec. 31, 2020
Dec. 31, 2019
Aug. 10, 2016
3.60% Senior Notes Due 2026 [Member]        
Debt Instrument [Line Items]        
Principal amount   $ 750 $ 750 $ 750
Exchange Offer Percentage 99.97%      
5.0% 2046 [Member]        
Debt Instrument [Line Items]        
Principal amount   $ 700 $ 700 $ 700
Exchange Offer Percentage 100.00%