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Nuclear Decommissioning Trust Fund (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Nuclear decommissioning trust fund disclosure      
Fair Value $ 692 $ 610  
Unrealized Gains 276 217  
Unrealized Losses 2 4  
Proceeds from sales of available-for-sale securities and the related realized gains and losses      
Realized gains 22 26 $ 21
Realized losses 8 11 14
Proceeds from sale of securities 501 510 $ 631
Cash and cash equivalents      
Nuclear decommissioning trust fund disclosure      
Fair Value 47 25  
Unrealized Gains 0 0  
Unrealized Losses $ 0 $ 0  
Weighted- average maturities (in years) 0 years 0 years  
U.S. government and federal agency obligations      
Nuclear decommissioning trust fund disclosure      
Fair Value $ 43 $ 73  
Unrealized Gains 1 1  
Unrealized Losses $ 0 $ 0  
Weighted- average maturities (in years) 11 years 11 years  
Federal agency mortgage-backed securities      
Nuclear decommissioning trust fund disclosure      
Fair Value $ 82 $ 62  
Unrealized Gains 1 1  
Unrealized Losses $ 1 $ 1  
Weighted- average maturities (in years) 23 years 25 years  
Commercial mortgage-backed securities      
Nuclear decommissioning trust fund disclosure      
Fair Value $ 13 $ 17  
Unrealized Gains 0 0  
Unrealized Losses $ 0 $ 1  
Weighted- average maturities (in years) 20 years 26 years  
Corporate debt securities      
Nuclear decommissioning trust fund disclosure      
Fair Value $ 99 $ 84  
Unrealized Gains 2 1  
Unrealized Losses $ 1 $ 2  
Weighted- average maturities (in years) 11 years 11 years  
Equity securities      
Nuclear decommissioning trust fund disclosure      
Fair Value $ 403 $ 346  
Unrealized Gains 272 214  
Unrealized Losses $ 0 $ 0  
Weighted- average maturities (in years) 0 years 0 years  
Foreign government fixed income securities      
Nuclear decommissioning trust fund disclosure      
Fair Value $ 5 $ 3  
Unrealized Gains 0 0  
Unrealized Losses $ 0 $ 0  
Weighted- average maturities (in years) 9 years 9 years  
South Texas Project      
Nuclear decommissioning trust fund disclosure      
Ownership Interest (as a percent) 44.00%