Investments (Schedule of Other than temporary Impairment Losses) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Gain (Loss) on Investments [Line Items]
Total other-than-temporary impairment losses	[1]	$ 29	$ 6	$ 55	$ 21
Losses recorded to nuclear decommissioning trust regulatory liability		(10)	(1)	(21)	(5)
Losses recognized in other comprehensive income (before taxes)		(3)	(1)	(7)	(3)
Net impairment losses recognized in earnings		16	4	27	13
Debt Securities
Gain (Loss) on Investments [Line Items]
Total other-than-temporary impairment losses		$ 3	$ 1	$ 7	$ 3

[1] Amounts include other-than-temporary impairment losses for debt securities of $3 million and $1 million for the three months ended September 31, 2015 and 2014, respectively, and $7 million and $3 million for the nine months ended September 31, 2015 and 2014, respectively.