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Investments (Marketable Equity and Debt Securities and Cash Equivalents (Classified as Available-for-sale) and Cost Method Investments in Decommissioning Trust Funds) (Detail) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Investment Holdings [Line Items] |  |  |
Amortized Cost, Total |  | $ 3,200 | $ 3,053
Total Unrealized Gains | [1] | 1,899 | 1,443
Total Unrealized Losses | [1],[2] | (6) | (12)
Fair Value |  | 5,093 | 4,484
Cash Equivalents and Other [Member] |  |  |
Investment Holdings [Line Items] |  |  |
Amortized Cost, Total | [3] | 34 | 12
Fair Value | [3] | 34 | 12
Cost Method Investments [Member] |  |  |
Investment Holdings [Line Items] |  |  |
Amortized Cost, Total |  | 68 | 69
Fair Value |  | 68 | 69
Common/collective trust funds |  |  |
Investment Holdings [Line Items] |  |  |
Amortized Cost, Total |  | 60 | 67
Fair Value |  | 60 | 67
Virginia Electric and Power Company |  |  |
Investment Holdings [Line Items] |  |  |
Amortized Cost, Total |  | 1,549 | 1,473
Total Unrealized Gains | [1] | 852 | 639
Total Unrealized Losses | [1],[4] | (2) | (6)
Fair Value |  | 2,399 | 2,106
Virginia Electric and Power Company | Cash Equivalents and Other [Member] |  |  |
Investment Holdings [Line Items] |  |  |
Amortized Cost, Other Investments | [5] | 22 | 7
Fair Value | [5] | 22 | 7
Virginia Electric and Power Company | Cost Method Investments [Member] |  |  |
Investment Holdings [Line Items] |  |  |
Amortized Cost, Other Investments |  | 68 | 69
Fair Value |  | 68 | 69
Virginia Electric and Power Company | Common/collective trust funds |  |  |
Investment Holdings [Line Items] |  |  |
Amortized Cost, Marketable Securities |  | 27 | 26
Fair Value |  | 27 | 26
Corporate debt instruments |  |  |
Investment Holdings [Line Items] |  |  |
Amortized Cost, Total |  | 430 | 478
Total Unrealized Gains | [1] | 15 | 13
Total Unrealized Losses | [1] | (1) | (4)
Fair Value |  | 444 | 487
Corporate debt instruments | Virginia Electric and Power Company |  |  |
Investment Holdings [Line Items] |  |  |
Amortized Cost, Marketable Securities |  | 216 | 274
Total Unrealized Gains | [1] | 8 | 6
Total Unrealized Losses | [1] |  | (4)
Fair Value |  | 224 | 276
Government Securities [Member] |  |  |
Investment Holdings [Line Items] |  |  |
Amortized Cost, Total |  | 1,039 | 978
Total Unrealized Gains | [1] | 27 | 22
Total Unrealized Losses | [1] | (5) | (8)
Fair Value |  | 1,061 | 992
Government Securities [Member] | Virginia Electric and Power Company |  |  |
Investment Holdings [Line Items] |  |  |
Amortized Cost, Marketable Securities |  | 482 | 420
Total Unrealized Gains | [1] | 13 | 9
Total Unrealized Losses | [1] | (2) | (2)
Fair Value |  | 493 | 427
U.S. | Equity securities: |  |  |
Investment Holdings [Line Items] |  |  |
Amortized Cost, Total |  | 1,569 | 1,449
Total Unrealized Gains | [1] | 1,857 | 1,408
Fair Value |  | 3,426 | 2,857
U.S. | Equity securities: | Virginia Electric and Power Company |  |  |
Investment Holdings [Line Items] |  |  |
Amortized Cost, Marketable Securities |  | 734 | 677
Total Unrealized Gains | [1] | 831 | 624
Fair Value |  | $ 1,565 | $ 1,301
[1] | Included in AOCI and the nuclear decommissioning trust regulatory liability as discussed in Note 2.
[2] | The fair value of securities in an unrealized loss position was $565 million and $576 million at December 31, 2017 and 2016, respectively.
[3] | Includes pending sales of securities of $5 million and $9 million at December 31, 2017 and 2016, respectively.
[4] | The fair value of securities in an unrealized loss position was $234 million and $287 million at December 31, 2017 and 2016, respectively.
[5] | Includes pending sales of securities of $6 million and $7 million at December 31, 2017 and 2016, respectively.

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