Derivatives and Hedge Accounting Activities (Schedule of Offsetting Liabilities) (Detail) - USD ($) $ in Millions		Sep. 30, 2021	Dec. 31, 2020
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[1]	$ 948	$ 485
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		69	46
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		531	17
Net Amounts		348	422
Virginia Electric and Power Company
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[2]	451	383
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		30	14
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		121	0
Net Amounts		300	369
Commodity | Over-the-counter
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[1]	148	30
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		10	9
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		79	0
Net Amounts		59	21
Commodity | Over-the-counter | Virginia Electric and Power Company
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[2]	123	6
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		7	6
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		79	0
Net Amounts		37	0
Commodity | Exchange
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[1]	473	24
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		32	24
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		441	0
Net Amounts		0	0
Commodity | Exchange | Virginia Electric and Power Company
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[2]	46	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		42	0
Net Amounts		0	0
Interest rate | Over-the-counter
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[1]	327	431
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		27	13
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		11	17
Net Amounts		289	401
Interest rate | Over-the-counter | Virginia Electric and Power Company
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[2]	282	376
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		19	7
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		0	0
Net Amounts		$ 263	$ 369

[1] Excludes $2 million and $1 million of derivative liabilities at September 30, 2021 and December 31, 2020, respectively, which are not subject to master netting or similar arrangements.

[2] Excludes $2 million and $22 million of derivative liabilities at September 30, 2021 and December 31, 2020, respectively, which are not subject to master netting or similar arrangements.