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Nuclear Decommissioning Trust Fund (Tables)
 12 Months Ended
Dec. 31, 2019
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, 2019As of December 31, 2018
(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$17  $—  $—  —  $19  $—  $—  —  
U.S. government and federal agency obligations
68  4  —  1146  1  —  12
Federal agency mortgage-backed securities
100  3  —  24100  1  2  23
Commercial mortgage-backed securities
29  1  1  2422  —  1  22
Corporate debt securities109  6  —  1196  1  2  11
Equity securities466  324  —  —  376  231  1  —  
Foreign government fixed income securities
5  —  —  104  —  —  9
Total$794  $338  $1   $663  $234  $6   
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)201920182017
Realized gains$18  $17  $22  
Realized (losses)(9) (13) (8) 
Proceeds from sale of securities381  513  501