Investments (Portion of Unrealized Gains and Losses Relates to Equity Securities) (Detail) - USD ($) $ in Millions		3 Months Ended	6 Months Ended
		Jun. 30, 2018	Jun. 30, 2018
Investment Holdings [Line Items]
Net gains recognized during the period		$ 89	$ 24
Less: Net gains recognized during the period on securities sold during the period		(16)	(35)
Unrealized gains (losses) recognized during the period on securities still held at June 30, 2018	[1]	73	(11)
Virginia Electric and Power Company
Investment Holdings [Line Items]
Net gains recognized during the period		44	12
Less: Net gains recognized during the period on securities sold during the period		(8)	(23)
Unrealized gains (losses) recognized during the period on securities still held at June 30, 2018	[1]	$ 36	$ (11)

[1] Included in other income and the nuclear decommissioning trust regulatory liability as discussed in Note 2.