Nuclear Decommissioning Trust Fund (Tables)

12 Months Ended

Dec. 31, 2016

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, 2016							As of December 31, 2015
(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	$	25	$	—	$	—	—	$	6	$	—	$	—	—
U.S. government and federal agency obligations	73		1		—		11	55		1		—		11
Federal agency mortgage-backed securities	62		1		1		25	59		1		—		25
Commercial mortgage-backed securities	17		—		1		26	25		—		2		28
Corporate debt securities	84		1		2		11	81		1		1		10
Equity securities	346		214		—		—	334		199		—		—
Foreign government fixed income securities	3		—		—		9	1		—		—		9
Total	$	610	$	217	$	4		$	561	$	202	$	3

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,
	2016		2015		2014
	(In millions)
Realized gains	$	26	$	21	$	29
Realized losses	11		14		8
Proceeds from sale of securities	510		631		600