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Long-Term Debt (Based on Stated Maturity Dates Rather than Early Redemption Dates that Could be Elected by Instrument Holders) (Parenthetical) (Detail) - USD ($) $ in Millions | 1 Months Ended | 12 Months Ended
--- | --- | ---
Feb. 28, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Debt Instrument [Line Items] |  |  |  |  |
Repayment of Debt |  |  | $ 1,572 | $ 1,610 | $ 892
Securities due within one year | [1],[2] |  | (3,078) | $ (1,709) |
Term Loans |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Estimated mandatory prepayments due within one year |  |  | 20 |  |
Dominion Energy Midstream Partners, LP |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Securities due within one year |  |  | $ 250 |  |
Dominion Energy Midstream Partners, LP | Subsequent Event |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Repayment of Debt |  | $ 250 |  |  |
3.53% Due 2028 | Dominion Energy Midstream Partners, LP | Private Placement | Subsequent Event |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Repayment of Debt |  | $ 100 |  |  |
Interest rate |  | 3.53% |  |  |
3.91% Due 2038 | Dominion Energy Midstream Partners, LP | Private Placement | Subsequent Event |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Repayment of Debt |  | $ 150 |  |  |
Interest rate |  | 3.91% |  |  |
[1] | Excludes $250 million of Dominion Energy Questar Pipeline's senior notes that matured in February 2018 which were repaid using proceeds from the January 2018 issuance, through private placement, of $100 million of 3.53% senior notes and $150 million of 3.91% senior notes that mature in 2028 and 2038, respectively.
[2] | Includes $20 million of estimated mandatory prepayments due within one year based on estimated cash flows in excess of debt service at SBL Holdco and Dominion Solar Projects III, Inc.

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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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