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Non-Derivative Fair Value Measurements (Details) - USD ($)
$ in Millions
 6 Months Ended 9 Months Ended 12 Months Ended
Jun. 30, 2021
Sep. 30, 2021
Dec. 31, 2020
Nuclear decommissinoning fund [Abstract]      
Equity investments in unconsolidated subsidiaries   $ 204 $ 165
Miscellaneous investments   158 154
Available-for-sale Securities, Gross Unrealized Gain $ 981 1,200  
Available-for-sale Securities, Gross Unrealized Loss   5 5
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]      
Due in one year or less   2  
Due in 1 to 5 years   153  
Due in 5 to 10 years   204  
Due after 10 years   299  
Total   658  
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost      
Nuclear decommissinoning fund [Abstract]      
Decommissioning Fund Investments   1,886 [1] 1,801 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Cash equivalents      
Nuclear decommissinoning fund [Abstract]      
Cash equivalents   38 [1] 40 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Commingled funds      
Nuclear decommissinoning fund [Abstract]      
Commingled funds   821 787
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Debt securities      
Nuclear decommissinoning fund [Abstract]      
Debt securities   620 [1] 528 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Equity securities      
Nuclear decommissinoning fund [Abstract]      
Equity Securities, FV-NI   407 [1] 446 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value      
Nuclear decommissinoning fund [Abstract]      
Alternative investment   1,208 [1] 1,041 [2]
Decommissioning Fund Investments   3,084 [1] 2,777 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Cash equivalents      
Nuclear decommissinoning fund [Abstract]      
Cash equivalents   38 [1] 40 [2]
Alternative investment   0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Commingled funds      
Nuclear decommissinoning fund [Abstract]      
Alternative investment   1,208 [1] 1,041 [2]
Commingled funds   1,208 1,041
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Debt securities      
Nuclear decommissinoning fund [Abstract]      
Debt securities   658 [1] 585 [2]
Alternative investment   0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Equity securities      
Nuclear decommissinoning fund [Abstract]      
Alternative investment   0 [1] 0 [2]
Equity Securities, FV-NI   1,180 [1] 1,111 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1      
Nuclear decommissinoning fund [Abstract]      
Decommissioning Fund Investments   1,216 [1] 1,149 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Cash equivalents      
Nuclear decommissinoning fund [Abstract]      
Cash equivalents   38 [1] 40 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Commingled funds      
Nuclear decommissinoning fund [Abstract]      
Commingled funds   0 0
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Debt securities      
Nuclear decommissinoning fund [Abstract]      
Debt securities   0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Equity securities      
Nuclear decommissinoning fund [Abstract]      
Equity Securities, FV-NI   1,178 [1] 1,109 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2      
Nuclear decommissinoning fund [Abstract]      
Decommissioning Fund Investments   645 [1] 574 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Cash equivalents      
Nuclear decommissinoning fund [Abstract]      
Cash equivalents   0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Commingled funds      
Nuclear decommissinoning fund [Abstract]      
Commingled funds   0 0
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Debt securities      
Nuclear decommissinoning fund [Abstract]      
Debt securities   643 [1] 572 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Equity securities      
Nuclear decommissinoning fund [Abstract]      
Equity Securities, FV-NI   2 [1] 2 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3      
Nuclear decommissinoning fund [Abstract]      
Decommissioning Fund Investments   15 [1] 13 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Cash equivalents      
Nuclear decommissinoning fund [Abstract]      
Cash equivalents   0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Commingled funds      
Nuclear decommissinoning fund [Abstract]      
Commingled funds   0 0
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Debt securities      
Nuclear decommissinoning fund [Abstract]      
Debt securities   15 [1] 13 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Equity securities      
Nuclear decommissinoning fund [Abstract]      
Equity Securities, FV-NI   $ 0 [1] $ 0 [2]
[1] Reported in nuclear decommissioning fund and other investments on the consolidated balance sheets, which also includes $204 million of equity method investments and $158 million of rabbi trust assets and miscellaneous investments.
[2] Reported in nuclear decommissioning fund and other investments on the consolidated balance sheets, which also includes $165 million of equity method investments and $154 million of rabbi trust assets and other miscellaneous investments.