Investments (Recorded Other-Than-Temporary Impairment Losses on Investments) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Investments, Debt and Equity Securities [Abstract]
Total other-than-temporary impairment losses	[1]	$ 9	$ 29	$ 34	$ 55
Losses recorded to the nuclear decommissioning trust regulatory liability		(4)	(10)	(15)	(21)
Losses recognized in other comprehensive income (before taxes)		0	(3)	(1)	(7)
Impairment		5	16	18	27
Other than temporary impairment losses for debt securities (less than $1 million for the three months ended September 30, 2016)		$ 1	$ 3	$ 2	$ 7

[1] Amounts include other-than-temporary impairment losses for debt securities of less than $1 million and $3 million for the three months ended September 30, 2016 and 2015, respectively, and $2 million and $7 million for the nine months ended September 30, 2016 and 2015, respectively.