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Long-Term Debt (IKE Zone Bonds) (Details) - Senior Notes [Member] - USD ($)
$ in Millions
1 Months Ended
Nov. 30, 2020
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2010
6.5% 2035 IKE Zone Senior Notes [Member]        
Debt Instrument [Line Items]        
Long-term Debt, Gross   $ 0 $ 65 $ 65
Extinguishment of Debt, Amount $ 65      
Debt Instrument, Redemption Price, Percentage of Principal Amount Redeemed 100.00%      
6 1/2% GO Zone Senior Notes Due 2035 [Member]        
Debt Instrument [Line Items]        
Long-term Debt, Gross   $ 0 $ 89 $ 89
Extinguishment of Debt, Amount $ 89      
Debt Instrument, Redemption Price, Percentage of Principal Amount Redeemed 100.00%