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Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Mar. 31, 2017 | Dec. 31, 2017 | Sep. 30, 2015
Change in Estimated Useful Life | Merchant generation assets |  |  |
Significant Accounting Policies [Line Items] |  |  |
Increase (decrease) in depreciation expense | $ (6) |  |
Increase (decrease) in depreciation expense, after tax | $ (4) |  |
Change in Estimated Useful Life | Scenario, Forecast | Merchant generation assets |  |  |
Significant Accounting Policies [Line Items] |  |  |
Increase (decrease) in depreciation expense |  | $ (26) |
Increase (decrease) in depreciation expense, after tax |  | (16) |
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Merchant Solar Projects |  |  |
Significant Accounting Policies [Line Items] |  |  |
Percentage of equity interest sold to noncontrolling interest owners | 33.00% |  | 33.00%
Four Brothers and Three Cedars |  |  |
Significant Accounting Policies [Line Items] |  |  |
Percentage ownership in total units | 50.00% |  |
Dominion Midstream Partners, LP | Common and Subordinated Units |  |  |
Significant Accounting Policies [Line Items] |  |  |
Ownership interest percentage of limited partner interests | 50.90% |  |
Dominion Midstream Partners, LP | Convertible Preferred Interests |  |  |
Significant Accounting Policies [Line Items] |  |  |
Ownership interest percentage of limited partner interests | 37.50% |  |
Virginia Electric and Power Company | Change in Depreciation Rates from New Depreciation Study |  |  |
Significant Accounting Policies [Line Items] |  |  |
Increase (decrease) in depreciation expense | $ 10 |  |
Increase (decrease) in depreciation expense, after tax | $ 6 |  |
Virginia Electric and Power Company | Change in Depreciation Rates from New Depreciation Study | Scenario, Forecast |  |  |
Significant Accounting Policies [Line Items] |  |  |
Increase (decrease) in depreciation expense |  | 40 |
Increase (decrease) in depreciation expense, after tax |  | $ 25 |

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- Definition The number of units or percentage investment held by one or more members or limited partners of the LLC or LP. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 272 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6373374&loc=d3e70478-108055 + Details
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