Derivatives and Hedge Accounting Activities (Schedule of Offsetting Liabilities) (Detail) - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[1]	$ 485	$ 735
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		46	53
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		17	35
Net Amounts		422	647
Virginia Electric and Power Company
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[2]	383	422
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		14	18
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		0	0
Net Amounts		369	404
Commodity | Over-the-counter
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[1]	30	105
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		9	21
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		0	0
Net Amounts		21	84
Commodity | Over-the-counter | Virginia Electric and Power Company
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[2]	6	59
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		6	18
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		0	0
Net Amounts		0	41
Commodity | Exchange
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[1]	24	21
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		24	21
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		0	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]	1	0
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		1	0
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		0	0
Net Amounts		0	0
Interest rate | Over-the-counter
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[1]	431	606
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		13	8
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		17	35
Net Amounts		401	563
Interest rate | Over-the-counter | Virginia Electric and Power Company
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[2]	376	363
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		7	0
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		0	0
Net Amounts		369	363
Foreign currency | Over-the-counter
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[1]	0	3
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		0	3
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		0	0
Net Amounts		$ 0	$ 0

[1] Excludes $1 million and $5 million of derivative liabilities at December 31, 2020 and 2019, respectively, which are not subject to master netting or similar arrangements.

[2] Excludes $22 million and $44 million of derivative liabilities at December 31, 2020 and 2019, respectively, which are not subject to master netting or similar arrangements.