Investments (Recorded Other-Than-Temporary Impairment Losses on Investments) (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Investments Debt And Equity Securities [Abstract]
Total other-than-temporary impairment losses	[1]	$ 8	$ 7	$ 25	$ 33
Losses recorded to the nuclear decommissioning trust regulatory liability			(2)		(13)
Losses recognized in other comprehensive income (before taxes)		$ (8)	(1)	$ (25)	(2)
Net impairment losses recognized in earnings			$ 4		$ 18

[1] Amounts include other-than-temporary impairment losses for debt securities of less than $1 million and $2 million for the three and nine months ended September 30, 2017, respectively.