Derivatives and Hedge Accounting Activities (Schedule of Offsetting Assets) (Detail) - USD ($) $ in Millions		Sep. 30, 2020	Dec. 31, 2019
Offsetting Assets [Line Items]
Gross Assets Presented in the Consolidated Balance Sheet	[1]	$ 250	$ 91
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		63	53
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received		0	0
Net Amounts		187	38
Virginia Electric and Power Company
Offsetting Assets [Line Items]
Gross Assets Presented in the Consolidated Balance Sheet	[2]	134	21
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		10	18
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received		0	0
Net Amounts		124	3
Commodity | Over-the-counter
Offsetting Assets [Line Items]
Gross Assets Presented in the Consolidated Balance Sheet	[1]	148	35
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		17	21
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received		0	0
Net Amounts		131	14
Commodity | Over-the-counter | Virginia Electric and Power Company
Offsetting Assets [Line Items]
Gross Assets Presented in the Consolidated Balance Sheet	[2]	134	19
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		10	18
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received		0	0
Net Amounts		124	1
Commodity | Exchange
Offsetting Assets [Line Items]
Gross Assets Presented in the Consolidated Balance Sheet	[1]	36	37
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		31	21
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received		0	0
Net Amounts		5	16
Interest rate | Over-the-counter
Offsetting Assets [Line Items]
Gross Assets Presented in the Consolidated Balance Sheet	[1]	60	11
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		9	3
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received		0	0
Net Amounts		51	8
Foreign currency | Over-the-counter
Offsetting Assets [Line Items]
Gross Assets Presented in the Consolidated Balance Sheet	[1]	6	8
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		6	8
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received		0	0
Net Amounts		0	0
Foreign currency | Over-the-counter | Virginia Electric and Power Company
Offsetting Assets [Line Items]
Gross Assets Presented in the Consolidated Balance Sheet	[2]	0	2
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		0	0
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received		0	0
Net Amounts		$ 0	$ 2

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

[2] Excludes $11 million and $3 million of derivative assets at September 30, 2020 and December 31, 2019, respectively, which are not subject to master netting or similar arrangements.