Nuclear Decommissioning Trust Fund (Tables)

3 Months Ended

Mar. 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 (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 March 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	$	16	$	—	$	—	—	$	47	$	—	$	—	—
U.S. government and federal agency obligations	56		1		1		11	43		1		—		11
Federal agency mortgage-backed securities	92		—		2		23	82		1		1		23
Commercial mortgage-backed securities	16		—		1		22	14		—		—		20
Corporate debt securities	100		1		2		11	99		2		1		11
Equity securities	395		265		—		—	402		272		—		—
Foreign government fixed income securities	5		—		—		8	5		—		—		9
Total	$	680	$	267	$	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 on the specific identification method.



	Three months ended March 31,
	2018		2017
	(In millions)
Realized gains	$	3	$	2
Realized losses	3		2
Proceeds from sale of securities	182		117