Derivatives and Hedge Accounting Activities (Schedule of Offsetting Liabilities) (Detail) - USD ($) $ in Millions		Sep. 30, 2020	Dec. 31, 2019
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[1]	$ 775	$ 735
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		63	53
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		20	35
Net Amounts		692	647
Virginia Electric and Power Company
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[2]	485	422
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		10	18
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		0	0
Net Amounts		475	404
Commodity | Over-the-counter
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[1]	19	105
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		17	21
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		0	0
Net Amounts		2	84
Commodity | Over-the-counter | Virginia Electric and Power Company
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[2]	10	59
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		10	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]	31	21
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		31	21
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]	721	606
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		11	8
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		20	35
Net Amounts		690	563
Interest rate | Over-the-counter | Virginia Electric and Power Company
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[2]	475	363
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		0	0
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral		0	0
Net Amounts		475	363
Foreign currency | Over-the-counter
Offsetting Liabilities [Line Items]
Gross Liabilities Presented in the Consolidated Balance Sheet	[1]	4	3
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		4	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 September 30, 2020 and December 31, 2019, respectively, which are not subject to master netting or similar arrangements.

[2] Excludes $6 million and $44 million of derivative liabilities at September 30, 2020 and December 31, 2019, respectively, which are not subject to master netting or similar arrangements.