Derivatives and Hedge Accounting Activities (Schedule of Offsetting Assets) (Detail) - USD ($) $ in Millions		Mar. 31, 2021	Dec. 31, 2020
Offsetting Assets [Line Items]
Gross Assets Presented in the Consolidated Balance Sheet	[1]	$ 849	$ 396
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		70	46
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received		0	0
Net Amounts		779	350
Virginia Electric and Power Company
Offsetting Assets [Line Items]
Gross Assets Presented in the Consolidated Balance Sheet	[2]	357	178
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		37	14
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received		0	0
Net Amounts		320	164
Commodity | Over-the-counter
Offsetting Assets [Line Items]
Gross Assets Presented in the Consolidated Balance Sheet	[1]	76	117
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		18	9
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received		0	0
Net Amounts		58	108
Commodity | Over-the-counter | Virginia Electric and Power Company
Offsetting Assets [Line Items]
Gross Assets Presented in the Consolidated Balance Sheet	[2]	71	111
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		15	6
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received		0	0
Net Amounts		56	105
Commodity | Exchange
Offsetting Assets [Line Items]
Gross Assets Presented in the Consolidated Balance Sheet	[1]	33	49
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		33	24
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received		0	0
Net Amounts		0	25
Commodity | Exchange | Virginia Electric and Power Company
Offsetting Assets [Line Items]
Gross Assets Presented in the Consolidated Balance Sheet	[2]	3	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 Received		0	0
Net Amounts		0	0
Interest rate | Over-the-counter
Offsetting Assets [Line Items]
Gross Assets Presented in the Consolidated Balance Sheet	[1]	740	230
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		19	13
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received		0	0
Net Amounts		721	217
Interest rate | Over-the-counter | Virginia Electric and Power Company
Offsetting Assets [Line Items]
Gross Assets Presented in the Consolidated Balance Sheet	[2]	283	66
Gross Amounts Not Offset in the Consolidated Balance Sheet, Financial Instruments		19	7
Gross Amounts Not Offset in the Consolidated Balance Sheet, Cash Collateral Received		0	0
Net Amounts		$ 264	$ 59

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

[2] Excludes $2 million and $3 million of derivative assets at March 31, 2021 and December 31, 2020, respectively, which are not subject to master netting or similar arrangements.