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Short-Term Debt and Credit Agreements (Narrative) (Detail) | 1 Months Ended | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Feb. 28, 2018 USD ($) | Mar. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) CreditFacility | Dec. 31, 2016 USD ($)
Debt Instrument [Line Items] |  |  |  |  |
Facility Limit |  |  |  | $ 5,500,000,000 | $ 5,500,000,000
Short-term debt |  |  |  | $ 3,298,000,000 | 3,155,000,000
Number of joint revolving credit facilities | CreditFacility |  |  |  | 2 |
Virginia Electric and Power Company |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Facility Limit | [1] |  |  | $ 5,500,000,000 | 5,500,000,000
Short-term debt |  |  |  | 542,000,000 | $ 65,000,000
Credit facility |  |  |  | 100,000,000 |
Variable rate tax-exempt financings |  |  |  | 100,000,000 |
Virginia Electric and Power Company | Subsequent Event |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Redemption of credit facility |  | $ 100,000,000 |  |  |
Joint Revolving Credit Facility 5 Billion and Joint Revolving Credit Facility 500 Million | Questar Gas |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Facility Limit |  |  |  | 250,000,000 |
Joint Revolving Credit Facility 5 Billion and Joint Revolving Credit Facility 500 Million | Virginia Electric and Power Company |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Facility Limit |  |  |  | 1,500,000,000 |
Credit Facilities, Maturing in December 2018 with 1 year Automatic Renewals through 2023 | SBL Holdco |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Facility Limit |  |  |  | $ 30,000,000 |
Automatic renewal period |  |  |  | 1 year |
Short-term debt |  |  |  | $ 0 |
Credit Facilities, Maturing in May 2018 with 1 year Automatic Renewals through 2024 | Dominion Solar Projects III, Inc |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Facility Limit |  |  |  | $ 25,000,000 |
Automatic renewal period |  |  |  | 1 year |
Short-term debt |  |  |  | $ 0 |
Six Billion Joint Revolving Credit Facility [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Maximum allowed total debt to total capital ratio |  |  |  | 65.00% |
Six Billion Joint Revolving Credit Facility [Member] | Scenario, Forecast |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Facility Limit |  |  | $ 6,000,000,000 |  |
Duration of credit facilities |  |  | 5 years |  |
Maximum allowed total debt to total capital ratio |  |  | 67.50% |  |
Term Loan Credit Agreement [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Duration of credit facilities |  | 364 days |  |  |
Maximum allowed total debt to total capital ratio |  | 67.50% |  |  |
Guarantee recorded amount |  | $ 950,000,000 |  |  |
[1] | The full amount of the facilities is available to Virginia Power, less any amounts outstanding to co-borrowers Dominion Energy, Dominion Energy Gas and Questar Gas. Sub-limits for Virginia Power are set within the facility limit but can be changed at the option of the Companies multiple times per year. At December 31, 2017, the sub-limit for Virginia Power was an aggregate $1.5 billion. If Virginia Power has liquidity needs in excess of its sub-limit, the sub-limit may be changed or such needs may be satisfied through short-term intercompany borrowings from Dominion Energy. These facilities mature in April 2020, and can be used to support bank borrowings and the issuance of commercial paper, as well as to support up to $2.0 billion (or the sub-limit, whichever is less) of letters of credit.

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- Definition The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the 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)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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