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Nuclear Decommissioning Trust Fund - Summary of Proceeds from Sale of Available -for-sale Securities (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Regulated Operations [Abstract]      
Realized gains $ 34 $ 18 $ 17
Realized (losses) (13) (9) (13)
Proceeds from sales of nuclear decommissioning trust fund securities $ 439 $ 381 $ 513