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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, 2018 | Dec. 31, 2017
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost, Total |  | $ 3,348 |  | $ 3,200 |
Total Unrealized Gains |  | 1,662 |  | 1,899 |
Total Unrealized Losses | [1] | (72) |  | (6) |
Fair Value, Total |  | 4,938 |  | 5,093 |
Fixed income securities Fair Value |  | 1,604 |  |  |
Equity securities Total Unrealized Losses |  | (245) |  |  |
Fixed Income |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Cash equivalents and other Amortized Cost |  | 4 |  | 34 |
Cash equivalents and other Fair Value |  | 4 |  | 34 |
Common/collective trust funds | Fixed Income |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Fixed income securities Amortized Cost, Total | [2] | 76 |  | 60 |
Fixed income securities Fair Value | [2] | 76 |  | 60 |
Cost Method Investments [Member] |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost, Total |  |  |  | 68 |
Fair Value, Total |  |  |  | 68 |
Equity securities: | U.S. |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Equity securities Amortized Cost, | [3] | 1,741 |  | 1,569 |
Equity securities Total Unrealized Gains | [3] | 1,640 |  | 1,857 |
Equity securities Total Unrealized Losses | [3] | (51) |  |  |
Equity securities Fair Value | [3] | 3,330 |  | 3,426 |
Corporate debt instruments | Fixed Income |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Fixed income securities Amortized Cost, Total | [2] | 435 |  | 430 |
Fixed income securities Total Unrealized Gains | [2] | 5 |  | 15 |
Fixed income securities Total Unrealized Losses | [2] | (9) |  | (1) |
Fixed income securities Fair Value | [2] | 431 |  | 444 |
Government Securities | Fixed Income |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Fixed income securities Amortized Cost, Total | [2] | 1,092 |  | 1,039 |
Fixed income securities Total Unrealized Gains | [2] | 17 |  | 27 |
Fixed income securities Total Unrealized Losses | [2] | (12) |  | (5) |
Fixed income securities Fair Value | [2] | 1,097 |  | 1,061 |
Virginia Electric and Power Company |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost, Total |  | 1,643 |  | 1,549 |
Total Unrealized Gains |  | 760 |  | 852 |
Total Unrealized Losses | [4] | (34) |  |  |
Fair Value, Total |  | 2,369 |  | 2,399 |
Fixed income securities Fair Value |  | 778 |  |  |
Equity securities Total Unrealized Losses |  | (105) |  |  |
Virginia Electric and Power Company | Common/collective trust funds | Fixed Income |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Fixed income securities Amortized Cost, Total | [2] | 51 |  | 27 |
Fixed income securities Fair Value | [2] | 51 |  | 27 |
Virginia Electric and Power Company | Cash Equivalents and Other [Member] |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Cash equivalents and other Amortized Cost | [5] | 6 |  | 22 |
Cash equivalents and other Fair Value |  | 6 |  | 22 |
Virginia Electric and Power Company | Cost Method Investments [Member] |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Amortized Cost, Total |  |  |  | 68 |
Fair Value, Total |  |  |  | 68 |
Virginia Electric and Power Company | Equity securities: | U.S. |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Equity securities Amortized Cost, |  | 858 | [3] | 734 | [2]
Equity securities Total Unrealized Gains |  | 751 | [3] | 831 | [2]
Equity securities Total Unrealized Losses | [3] | (24) |  |  |
Equity securities Fair Value |  | 1,585 | [3] | 1,565 | [2]
Virginia Electric and Power Company | Corporate debt instruments | Fixed Income |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Fixed income securities Amortized Cost, Total | [2] | 224 |  | 216 |
Fixed income securities Total Unrealized Gains | [2] | 2 |  | 8 |
Fixed income securities Fair Value | [2] | 221 |  | 224 |
Virginia Electric and Power Company | Government Securities | Fixed Income |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |
Fixed income securities Amortized Cost, Total | [2] | 504 |  | 482 |
Fixed income securities Total Unrealized Gains | [2] | 7 |  | 13 |
Fixed income securities Fair Value | [2] | $ 506 |  | $ 493 |
[1] | The fair value of securities in an unrealized loss position was $833 million and $565 million at December 31, 2018 and 2017, respectively.
[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.
[3] | 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.
[4] | The fair value of securities in an unrealized loss position was $404 million and $234 million at December 31, 2018 and 2017, respectively.
[5] | Includes pending sales of securities of $6 million at both December 31, 2018 and 2017.

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