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Investments (Equity and Debt Securities and Cash Equivalents and Cost Method Investments in Decommissioning Trust Funds) (Detail) - USD ($) $ in Millions | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost, Total |  | $ 3,243 |  | $ 3,200 |
Total Unrealized Gains |  | 1,850 |  | 1,899 |
Total Unrealized Losses | [1] | (33) |  | (6) |
Fair Value |  | 5,060 |  | 5,093 |
Virginia Electric and Power Company |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost, Total |  | 1,575 |  | 1,549 |
Total Unrealized Gains |  | 841 |  | 852 |
Total Unrealized Losses | [2] | (16) |  | (2) |
Fair Value |  | 2,400 |  | 2,399 |
Cash Equivalents and Other [Member] |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost, Total | [3] | 8 |  | 34 |
Fair Value | [3] | 8 |  | 34 |
Cash Equivalents and Other [Member] | Virginia Electric and Power Company |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost, Other Investments | [4] | 7 |  | 22 |
Fair Value | [4] | 7 |  | 22 |
Cost-method Investments |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost, Total |  |  |  | 68 |
Fair Value |  |  |  | 68 |
Cost-method Investments | Virginia Electric and Power Company |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost, Other Investments |  |  |  | 68 |
Fair Value |  |  |  | 68 |
Common/collective trust funds | Fixed Income Securities |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost, Total | [5] | 56 |  | 60 |
Fair Value | [5] | 56 |  | 60 |
Common/collective trust funds | Fixed Income Securities | Virginia Electric and Power Company |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost, Total | [6] | 25 |  | 27 |
Fair Value | [6] | 25 |  | 27 |
Corporate debt instruments | Fixed Income Securities |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost, Total | [5] | 442 |  | 430 |
Total Unrealized Gains | [5] | 9 |  | 15 |
Total Unrealized Losses | [5] | (6) |  | (1) |
Fair Value | [5] | 445 |  | 444 |
Corporate debt instruments | Fixed Income Securities | Virginia Electric and Power Company |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost, Total | [6] | 219 |  | 216 |
Total Unrealized Gains | [6],[7] | 4 |  | 8 |
Total Unrealized Losses | [6],[7] | (2) |  |  |
Fair Value | [6] | 221 |  | 224 |
Government securities | Fixed Income Securities |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost, Total | [5] | 1,061 |  | 1,039 |
Total Unrealized Gains | [5] | 17 |  | 27 |
Total Unrealized Losses | [5] | (12) |  | (5) |
Fair Value | [5] | 1,066 |  | 1,061 |
Government securities | Fixed Income Securities | Virginia Electric and Power Company |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost, Total | [6] | 496 |  | 482 |
Total Unrealized Gains | [6],[7] | 8 |  | 13 |
Total Unrealized Losses | [6],[7] | (5) |  | (2) |
Fair Value | [6] | 499 |  | 493 |
Equity securities: | U.S. |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost, Total |  | 1,676 | [8] | 1,569 | [5]
Total Unrealized Gains |  | 1,824 | [8] | 1,857 | [5]
Total Unrealized Losses | [8] | (15) |  |  |
Fair Value |  | 3,485 | [8] | 3,426 | [5]
Equity securities: | U.S. | Virginia Electric and Power Company |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost, Total | [8] | 828 |  | 734 |
Total Unrealized Gains | [7],[8] | 829 |  | 831 |
Total Unrealized Losses | [7],[8] | (9) |  |  |
Fair Value | [8] | $ 1,648 |  | $ 1,565 |
[1] | The fair value of securities in an unrealized loss position was $1.1 billion and $565 million at March 31, 2018 and December 31, 2017, respectively.
[2] | The fair value of securities in an unrealized loss position was $510 million and $234 million at March 31, 2018 and December 31, 2017, respectively.
[3] | Includes pending sales of securities of $3 million and $5 million at March 31, 2018 and December 31, 2017, respectively.
[4] | Includes pending sales of securities of $4 million and $6 million at March 31, 2018 and December 31, 2017, respectively.
[5] | Unrealized gains and losses on equity securities (for 2017) and fixed income securities are included in AOCI and the nuclear decommissioning trust regulatory liability as discussed in Note 2.
[6] | Unrealized gains and losses on equity securities (for 2017) and fixed income securities are included in AOCI and the nuclear decommissioning trust regulatory liability as discussed in Note 2.
[7] | Effective January 2018, unrealized gains and losses on equity securities, including those previously classified as cost method investments, are included in other income and the nuclear decommissioning trust regulatory liability as discussed in Note 2. Unrealized gains and losses on equity securities (for 2017) and fixed income securities are included in AOCI and the nuclear decommissioning trust regulatory liability as discussed in Note 2.
[8] | Effective January 2018, unrealized gains and losses on equity securities, including those previously classified as cost method investments, are included in other income and the nuclear decommissioning trust regulatory liability as discussed in Note 2.

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