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Investments (Equity and Fixed Income Securities, Insurance Contracts and Cash Equivalents in Decommissioning Trust Funds) (Detail) - USD ($) | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Sep. 30, 2020 | Sep. 30, 2020 | Dec. 31, 2019 | Jun. 30, 2020
Investment Holdings [Line Items] |  |  |  |  |
Fixed income securities Fair Value |  | $ 1,950,000,000 | $ 1,950,000,000 |  |
Amortized Cost, Total |  | 3,798,000,000 | 3,798,000,000 | $ 3,691,000,000 |
Total Unrealized Gains |  | 2,635,000,000 | 2,635,000,000 | 2,523,000,000 |
Total Unrealized Losses | [1] | (76,000,000) | (76,000,000) | (22,000,000) |
Allowance for Credit Losses, Total |  | 0 | 0 | 0 | $ 0
Fair Value, Total |  | 6,357,000,000 | 6,357,000,000 | 6,192,000,000 |
Net assets related to pending sales and purchases of securities |  | 36,000,000 | 36,000,000 | 1,000,000 |
Fair value of securities in an unrealized loss position |  | 523,000,000 | 523,000,000 | 298,000,000 |
Virginia Electric and Power Company |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Fixed income securities Fair Value |  | 882,000,000 | 882,000,000 |  |
Amortized Cost, Total |  | 1,770,000,000 | 1,770,000,000 | 1,721,000,000 |
Total Unrealized Gains |  | 1,212,000,000 | 1,212,000,000 | 1,173,000,000 |
Total Unrealized Losses | [2] | (49,000,000) | (49,000,000) | (13,000,000) |
Allowance for Credit Losses, Total |  | 0 | 0 | 0 | $ 0
Fair Value, Total |  | 2,933,000,000 | 2,933,000,000 | 2,881,000,000 |
Net assets related to pending sales and purchases of securities |  | 5,000,000 | 5,000,000 |  |
Fair value of securities in an unrealized loss position |  | 283,000,000 | 283,000,000 | 185,000,000 |
Decrease in allowance for credit loss |  | 12,000,000 |  |  |
Fixed Income |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Allowance for Credit Losses, Total |  | 21,000,000 | 21,000,000 |  |
Common/collective trust funds | Fixed Income |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Fixed income securities Amortized Cost, Total | [3] | 138,000,000 | 138,000,000 | 115,000,000 |
Fixed income securities Total Unrealized Gains | [3] |  | 4,000,000 | 4,000,000 |
Fixed income securities Total Unrealized Losses | [3] |  | 0 | 0 |
Fixed income securities Allowance for Credit Losses | [3] | 0 | 0 | 0 |
Fixed income securities Fair Value | [3] | 142,000,000 | 142,000,000 | 119,000,000 |
Common/collective trust funds | Fixed Income | Virginia Electric and Power Company |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Fixed income securities Amortized Cost, Total | [3] | 45,000,000 | 45,000,000 | 51,000,000 |
Fixed income securities Total Unrealized Gains | [3] |  | 0 | 0 |
Fixed income securities Total Unrealized Losses | [3] |  | 0 | 0 |
Fixed income securities Allowance for Credit Losses | [3] | 0 | 0 | 0 |
Fixed income securities Fair Value | [3] | 45,000,000 | 45,000,000 | 51,000,000 |
Equity securities: | U.S. |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Equity securities Amortized Cost | [4] | 1,735,000,000 | 1,735,000,000 | 1,807,000,000 |
Equity securities Total Unrealized Gains | [4] |  | 2,517,000,000 | 2,451,000,000 |
Equity securities Total Unrealized Losses | [4] |  | (73,000,000) | (20,000,000) |
Equity securities Allowance for Credit Losses | [4] | 0 | 0 | 0 |
Equity securities Fair Value | [4] | 4,179,000,000 | 4,179,000,000 | 4,238,000,000 |
Equity securities: | U.S. | Virginia Electric and Power Company |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Equity securities Amortized Cost | [4] | 919,000,000 | 919,000,000 | 894,000,000 |
Equity securities Total Unrealized Gains | [4] |  | 1,161,000,000 | 1,144,000,000 |
Equity securities Total Unrealized Losses | [4] |  | (48,000,000) | (11,000,000) |
Equity securities Allowance for Credit Losses | [4] | 0 | 0 | 0 |
Equity securities Fair Value | [4] | 2,032,000,000 | 2,032,000,000 | 2,027,000,000 |
Corporate debt instruments | Fixed Income |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Fixed income securities Amortized Cost, Total | [3] | 569,000,000 | 569,000,000 | 434,000,000 |
Fixed income securities Total Unrealized Gains | [3] |  | 49,000,000 | 29,000,000 |
Fixed income securities Total Unrealized Losses | [3] |  | (1,000,000) | 0 |
Fixed income securities Allowance for Credit Losses | [3] | 0 | 0 | 0 |
Fixed income securities Fair Value | [3] | 617,000,000 | 617,000,000 | 463,000,000 |
Corporate debt instruments | Fixed Income | Virginia Electric and Power Company |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Fixed income securities Amortized Cost, Total | [3] | 324,000,000 | 324,000,000 | 241,000,000 |
Fixed income securities Total Unrealized Gains | [3] |  | 28,000,000 | 15,000,000 |
Fixed income securities Total Unrealized Losses | [3] |  | 0 | 0 |
Fixed income securities Allowance for Credit Losses | [3] | 0 | 0 | 0 |
Fixed income securities Fair Value | [3] | 352,000,000 | 352,000,000 | 256,000,000 |
Government Securities | Fixed Income |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Fixed income securities Amortized Cost, Total | [3] | 1,128,000,000 | 1,128,000,000 | 1,108,000,000 |
Fixed income securities Total Unrealized Gains | [3] |  | 64,000,000 | 39,000,000 |
Fixed income securities Total Unrealized Losses | [3] |  | (1,000,000) | (2,000,000) |
Fixed income securities Allowance for Credit Losses | [3] | 0 | 0 | 0 |
Fixed income securities Fair Value | [3] | 1,191,000,000 | 1,191,000,000 | 1,145,000,000 |
Government Securities | Fixed Income | Virginia Electric and Power Company |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Fixed income securities Amortized Cost, Total | [3] | 463,000,000 | 463,000,000 | 534,000,000 |
Fixed income securities Total Unrealized Gains | [3] |  | 23,000,000 | 14,000,000 |
Fixed income securities Total Unrealized Losses | [3] |  | (1,000,000) | (2,000,000) |
Fixed income securities Allowance for Credit Losses | [3] | 0 | 0 | 0 |
Fixed income securities Fair Value | [3] | 485,000,000 | 485,000,000 | 546,000,000 |
Insurance Contracts |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Fixed income securities Amortized Cost, Total |  | 227,000,000 | 227,000,000 | 214,000,000 |
Fixed income securities Total Unrealized Gains |  |  | 0 | 0 |
Fixed income securities Total Unrealized Losses |  |  | 0 | 0 |
Fixed income securities Allowance for Credit Losses |  | 0 | 0 | 0 |
Fixed income securities Fair Value |  | 227,000,000 | 227,000,000 | 214,000,000 |
Cash equivalents and other |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Cash equivalents and other, Amortized Cost |  | 1,000,000 | 1,000,000 | 13,000,000 |
Cash equivalents and other, Total Unrealized Gains |  | 1,000,000 | 1,000,000 |  |
Cash equivalents and other, Total Unrealized Losses |  | (1,000,000) | (1,000,000) |  |
Cash equivalents and other, Allowance for Credit Losses |  | 0 | 0 | 0 |
Cash equivalents and other, Fair Value |  | 1,000,000 | 1,000,000 | 13,000,000 |
Cash equivalents and other | Virginia Electric and Power Company |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Cash equivalents and other, Amortized Cost |  | 19,000,000 | 19,000,000 | 1,000,000 |
Cash equivalents and other, Allowance for Credit Losses |  | 0 | 0 | 0 |
Cash equivalents and other, Fair Value |  | $ 19,000,000 | $ 19,000,000 | $ 1,000,000 |
[1] | The allowance for credit losses associated with fixed income securities decreased from March 31, 2020 by $21 million. These recoveries are a result of improvements in credit spreads experienced in the market between March 31, 2020 and June 30, 2020. There were no further changes in the allowance for credit losses between June 30, 2020 and September 30, 2020.
[2] | The allowance for credit losses associated with fixed income securities decreased from March 31, 2020 by $12 million. These recoveries are a result of improvements in credit spreads experienced in the market between March 31, 2020 and June 30, 2020. There were no further changes in the allowance for credit losses between June 30, 2020 and September 30, 2020.
[3] | Unrealized gains and losses on fixed income securities are included in AOCI and the nuclear decommissioning trust regulatory liability. Effective January 2020, changes in allowance for credit losses are included in other income.
[4] | Unrealized gains and losses on equity securities are included in other income and the nuclear decommissioning trust regulatory liability.

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