Derivatives and Hedge Accounting Activities (Schedule of Offsetting Assets) (Detail) - USD ($) $ in Millions		Sep. 30, 2021	Dec. 31, 2020
Offsetting Assets [Line Items]
Gross Assets Presented in the Consolidated Balance Sheet	[1]	$ 618	$ 396
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		69	46
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received		0	0
Net Amounts		549	350
Virginia Electric and Power Company
Offsetting Assets [Line Items]
Gross Assets Presented in the Consolidated Balance Sheet	[2]	296	178
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		30	14
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received		0	0
Net Amounts		266	164
Commodity | Over-the-counter
Offsetting Assets [Line Items]
Gross Assets Presented in the Consolidated Balance Sheet	[1]	202	117
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		10	9
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received		0	0
Net Amounts		192	108
Commodity | Over-the-counter | Virginia Electric and Power Company
Offsetting Assets [Line Items]
Gross Assets Presented in the Consolidated Balance Sheet	[2]	122	111
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		7	6
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received		0	0
Net Amounts		115	105
Commodity | Exchange
Offsetting Assets [Line Items]
Gross Assets Presented in the Consolidated Balance Sheet	[1]	45	49
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		32	24
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received		0	0
Net Amounts		13	25
Commodity | Exchange | Virginia Electric and Power Company
Offsetting Assets [Line Items]
Gross Assets Presented in the Consolidated Balance Sheet	[2]	4	1
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		4	1
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received		0	0
Net Amounts		0	0
Interest rate | Over-the-counter
Offsetting Assets [Line Items]
Gross Assets Presented in the Consolidated Balance Sheet	[1]	371	230
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		27	13
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received		0	0
Net Amounts		344	217
Interest rate | Over-the-counter | Virginia Electric and Power Company
Offsetting Assets [Line Items]
Gross Assets Presented in the Consolidated Balance Sheet	[2]	170	66
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		19	7
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received		0	0
Net Amounts		$ 151	$ 59

[1] Excludes $38 million and $1 million of derivative assets at September 30, 2021 and December 31, 2020, respectively, which are not subject to master netting or similar arrangements.

[2] Excludes $63 million and $3 million of derivative assets at September 30, 2021 and December 31, 2020, respectively, which are not subject to master netting or similar arrangements.