Nuclear Decommissioning Trust Fund (Tables)

6 Months Ended

Jun. 30, 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 June 30, 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	$	11	$	—	$	—	—	$	25	$	—	$	—	—
U.S. government and federal agency obligations	45		2		—		11	73		1		—		11
Federal agency mortgage-backed securities	71		1		1		24	62		1		1		25
Commercial mortgage-backed securities	17		—		—		24	17		—		1		26
Corporate debt securities	109		2		1		10	84		1		2		11
Equity securities	380		245		—		—	346		214		—		—
Foreign government fixed income securities	4		—		—		7	3		—		—		9
Total	$	637	$	250	$	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 on the specific identification method.

	Six months ended June 30,
	2017		2016
	(In millions)
Realized gains	$	3	$	3
Realized losses	3		2
Proceeds from sale of securities	277		267