Derivatives and Hedge Accounting Activities (Schedule of Offsetting Liabilities) (Detail) - USD ($) $ in Millions		Mar. 31, 2021	Dec. 31, 2020
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[1]	$ 264	$ 485
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		70	46
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		28	17
Net Amounts		166	422
Virginia Electric and Power Company
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[2]	173	383
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		37	14
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		1	0
Net Amounts		135	369
Commodity | Over-the-counter
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[1]	28	30
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		18	9
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		0	0
Net Amounts		10	21
Commodity | Over-the-counter | Virginia Electric and Power Company
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[2]	19	6
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		15	6
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		0	0
Net Amounts		4	0
Commodity | Exchange
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[1]	50	24
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		33	24
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		17	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]	4	1
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		3	1
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		1	0
Net Amounts		0	0
Interest rate | Over-the-counter
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[1]	186	431
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		19	13
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		11	17
Net Amounts		156	401
Interest rate | Over-the-counter | Virginia Electric and Power Company
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[2]	150	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		$ 131	$ 369

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

[2] Excludes $5 million and $22 million of derivative liabilities at March 31, 2021 and December 31, 2020, respectively, which are not subject to master netting or similar arrangements.