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Investments (Equity and Debt Securities and Cash Equivalents and Cost Method Investments in Decommissioning Trust Funds) (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost, Total |  | $ 3,691 |  | $ 3,348 |
Total Unrealized Gains |  | 2,523 |  | 1,662 |
Total Unrealized Losses | [1] | (22) |  | (72) |
Fair Value, Total |  | 6,192 |  | 4,938 |
Fixed income securities Fair Value |  | 1,727 |  |  |
Fixed Income |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Cash equivalents and other Amortized Cost |  | 13 | [2] | 4 |
Cash equivalents and other Fair Value |  | 13 | [2] | 4 |
Common/collective trust funds | Fixed Income |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Fixed income securities Amortized Cost, Total | [3] | 115 |  | 76 |
Fixed income securities Total Unrealized Gains |  | 4 | [3] | 0 |
Fixed income securities Fair Value | [3] | 119 |  | 76 |
Equity securities: | U.S. |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Equity securities Amortized Cost, | [4] | 1,807 | [3] | 1,741 |
Equity securities Total Unrealized Gains | [4] | 2,451 | [3] | 1,640 |
Equity securities Total Unrealized Losses | [4] | (20) | [3] | (51) |
Equity securities Fair Value | [4] | 4,238 | [3] | 3,330 |
Corporate debt instruments | Fixed Income |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Fixed income securities Amortized Cost, Total | [3] | 434 |  | 435 |
Fixed income securities Total Unrealized Gains | [3] | 29 |  | 5 |
Fixed income securities Total Unrealized Losses | [3] |  |  | (9) |
Fixed income securities Fair Value | [3] | 463 |  | 431 |
Government Securities | Fixed Income |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Fixed income securities Amortized Cost, Total | [3] | 1,108 |  | 1,092 |
Fixed income securities Total Unrealized Gains | [3] | 39 |  | 17 |
Fixed income securities Total Unrealized Losses | [3] | (2) |  | (12) |
Fixed income securities Fair Value | [3] | 1,145 |  | 1,097 |
Insurance contracts |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost, Total |  | 214 |  |  |
Fair Value, Total |  | 214 |  |  |
Virginia Electric and Power Company |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost, Total |  | 1,721 |  | 1,643 |
Total Unrealized Gains |  | 1,173 |  | 760 |
Total Unrealized Losses |  | (13) | [1],[5] | (34) |
Fair Value, Total |  | 2,881 |  | 2,369 |
Fixed income securities Fair Value |  | 853 |  |  |
Virginia Electric and Power Company | Common/collective trust funds | Fixed Income |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Fixed income securities Amortized Cost, Total | [3] | 51 |  | 51 |
Fixed income securities Fair Value | [3] | 51 |  | 51 |
Virginia Electric and Power Company | Cash Equivalents and Other [Member] |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Cash equivalents and other Amortized Cost |  | 1 |  | 6 | [3],[6]
Cash equivalents and other Fair Value |  | 1 |  | 6 | [3],[6]
Virginia Electric and Power Company | Equity securities: | U.S. |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Equity securities Amortized Cost, |  | 894 | [4] | 858 |
Equity securities Total Unrealized Gains |  | 1,144 | [4] | 751 |
Equity securities Total Unrealized Losses |  | (11) | [4] | (24) |
Equity securities Fair Value |  | 2,027 | [4] | 1,585 |
Virginia Electric and Power Company | Corporate debt instruments | Fixed Income |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Fixed income securities Amortized Cost, Total | [3] | 241 |  | 224 |
Fixed income securities Total Unrealized Gains | [3] | 15 |  | 2 |
Fixed income securities Total Unrealized Losses | [3] |  |  | (5) |
Fixed income securities Fair Value | [3] | 256 |  | 221 |
Virginia Electric and Power Company | Government Securities | Fixed Income |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Fixed income securities Amortized Cost, Total | [3] | 534 |  | 504 |
Fixed income securities Total Unrealized Gains | [3] | 14 |  | 7 |
Fixed income securities Total Unrealized Losses | [3] | (2) |  | (5) |
Fixed income securities Fair Value | [3] | $ 546 |  | $ 506 |
[1] | The fair value of securities in an unrealized loss position was $298 million and $833 million at December 31, 2019 and 2018, respectively.
[2] | Includes pending purchases of securities of $1 million at December 31, 2019.
[3] | Unrealized gains and losses on fixed income securities are included in AOCI and the nuclear decommissioning trust regulatory liability as discussed in Note 2.
[4] | Unrealized gains and losses on equity securities are included in other income and the nuclear decommissioning trust regulatory liability as discussed in Note 2.
[5] | The fair value of securities in an unrealized loss position was $185 million and $404 million at December 31, 2019 and 2018, respectively.
[6] | Includes pending sales of securities of $6 million at December 31, 2018.

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