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Nuclear Decommissioning Trust Fund (Tables)
 12 Months Ended
Dec. 31, 2020
Regulated Operations [Abstract]  
Summary of Aggregate Fair Values and Unrealized Gains And Losses (Including Other-Than-Temporary Impairments) for the Securities Held in The Nuclear Decommissioning Trust Fund
The following table summarizes the aggregate fair values and unrealized gains and losses for the securities held in the trust funds, as well as information about the contractual maturities of those securities.
 As of December 31, 2020As of December 31, 2019
(In millions, except otherwise noted)
Fair
Value
Unrealized
Gains
Unrealized
Losses
Weighted-
average
maturities
(in years)
Fair
Value
Unrealized
Gains
Unrealized
Losses
Weighted-
average
maturities
(in years)
Cash and cash equivalents$23 $— $— — $17 $— $— — 
U.S. government and federal agency obligations
70 6 — 1068 4 — 11
Federal agency mortgage-backed securities
89 4 — 24100 3 — 24
Commercial mortgage-backed securities
36 2 — 2729 1 1 24
Corporate debt securities144 13 — 12109 6 — 11
Equity securities521 372 — — 466 324 — — 
Foreign government fixed income securities
7 1 — 105 — — 10
Total$890 $398 $—  $794 $338 $1  
Summary of Proceeds From Sales of Available-For-Sale securities and the related realized gains and losses
The following table summarizes proceeds from sales of available-for-sale securities and the related realized gains and losses from these sales. The cost of securities sold is determined using the specific identification method.
 Year Ended December 31,
(In millions)202020192018
Realized gains$34 $18 $17 
Realized (losses)(13)(9)(13)
Proceeds from sale of securities439 381 513