Derivatives and Hedge Accounting Activities (Schedule of Offsetting Liabilities) (Detail) - USD ($) $ in Millions		Jun. 30, 2021	Dec. 31, 2020
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[1]	$ 546	$ 485
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		57	46
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		170	17
Net Amounts		319	422
Virginia Electric and Power Company
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[2]	333	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		16	0
Net Amounts		287	369
Commodity | Over-the-counter
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[1]	49	30
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		13	9
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		9	0
Net Amounts		27	21
Commodity | Over-the-counter | Virginia Electric and Power Company
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[2]	42	6
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		10	6
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		9	0
Net Amounts		23	0
Commodity | Exchange
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[1]	168	24
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		18	24
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		150	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]	8	1
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		1	1
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		7	0
Net Amounts		0	0
Interest rate | Over-the-counter
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[1]	329	431
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		26	13
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		11	17
Net Amounts		292	401
Interest rate | Over-the-counter | Virginia Electric and Power Company
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[2]	283	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		$ 264	$ 369

[1] Excludes $28 million and $1 million of derivative liabilities at June 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 June 30, 2021 and December 31, 2020, respectively, which are not subject to master netting or similar arrangements.