Nuclear Decommissioning Trust Fund (Tables)

12 Months Ended

Dec. 31, 2017

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 (including other-than-temporary impairments) for the securities held in the trust funds, as well as information about the contractual maturities of those securities.



	As of December 31, 2017							As of December 31, 2016
(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	$	47	$	—	$	—	—	$	25	$	—	$	—	—
U.S. government and federal agency obligations	43		1		—		11	73		1		—		11
Federal agency mortgage-backed securities	82		1		1		23	62		1		1		25
Commercial mortgage-backed securities	13		—		—		20	17		—		1		26
Corporate debt securities	99		2		1		11	84		1		2		11
Equity securities	403		272		—		—	346		214		—		—
Foreign government fixed income securities	5		—		—		9	3		—		—		9
Total	$	692	$	276	$	2		$	610	$	217	$	4

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,
	2017		2016		2015
	(In millions)
Realized gains	$	22	$	26	$	21
Realized losses	8		11		14
Proceeds from sale of securities	501		510		631