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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.

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