Nuclear Decommissioning Trust Fund (Tables)

12 Months Ended

Dec. 31, 2018

Nuclear Decommissioning Trust Fund Disclosure [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, 2018							As of December 31, 2017
(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	$	19	$	—	$	—	—	$	47	$	—	$	—	—
U.S. government and federal agency obligations	46		1		—		12	43		1		—		11
Federal agency mortgage-backed securities	100		1		2		23	82		1		1		23
Commercial mortgage-backed securities	22		—		1		22	14		—		—		20
Corporate debt securities	96		1		2		11	99		2		1		11
Equity securities	376		231		1		—	402		272		—		—
Foreign government fixed income securities	4		—		—		9	5		—		—		9
Total	$	663	$	234	$	6		$	692	$	276	$	2

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,
	2018			2017			2016
	(In millions)
Realized gains	$	17		$	22		$	26
Realized (losses)	(13		)	(8		)	(11		)
Proceeds from sale of securities	513			501			510