Equity (Accumulated Other Comprehensive Income (Loss)) (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Accumulated Other Comprehensive Income Loss [Line Items]
Total AOCI, including noncontrolling interests	$ (1,717)	$ (1,793)
Accumulated other comprehensive loss	(1,717)	(1,793)
Deferred Gains and Losses on Derivatives-Hedging Activities
Accumulated Other Comprehensive Income Loss [Line Items]
Total AOCI, including noncontrolling interests	(419)	(407)
Unrealized Gains and Losses on Investment Securities
Accumulated Other Comprehensive Income Loss [Line Items]
Total AOCI, including noncontrolling interests	62	37
Unrecognized Pension and Other Postretirement Benefit Costs
Accumulated Other Comprehensive Income Loss [Line Items]
Total AOCI, including noncontrolling interests	(1,359)	(1,421)
Other Comprehensive Loss From Equity Method Investees
Accumulated Other Comprehensive Income Loss [Line Items]
Total AOCI, including noncontrolling interests	(1)	(2)
Virginia Electric and Power Company
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive loss	(52)	(29)
Virginia Electric and Power Company | Deferred Gains and Losses on Derivatives-Hedging Activities
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive loss	(60)	(34)
Virginia Electric and Power Company | Unrealized Gains and Losses on Investment Securities
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive loss	$ 8	$ 5