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Income Taxes (Unrecognized Tax Benefits) (Detail) - USD ($) $ in Millions | 1 Months Ended | 12 Months Ended
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Jun. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |  |
Balance at January 1 |  | $ 64 | $ 103 | $ 145
Increases-prior period positions |  | 1 | 9 | 2
Decreases-prior period positions |  | (9) | (44) | (40)
Increases-current period positions |  | 5 | 6 | 8
Settlements with tax authorities |  | (23) | (8) | (5)
Expiration of statutes of limitations |  |  | (2) | (7)
Balance at December 31 |  | 38 | 64 | 103
Virginia Electric and Power Company |  |  |  |
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |  |
Balance at January 1 |  | 13 | 12 | 36
Increases-prior period positions |  |  | 4 |
Decreases-prior period positions |  | (1) | (3) | (25)
Increases-current period positions |  |  |  | 1
Settlements with tax authorities | $ (8) | (8) |  |
Balance at December 31 |  | 4 | 13 | 12
Dominion Energy Gas Holdings, LLC |  |  |  |
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |  |
Balance at January 1 |  | 7 | 29 | 29
Increases-prior period positions |  |  | 1 |
Decreases-prior period positions |  |  | (19) |
Settlements with tax authorities |  | $ (7) | (4) |
Balance at December 31 |  |  | $ 7 | $ 29

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- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
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- Definition Amount of unrecognized tax benefits. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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- Definition Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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- Definition Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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- Definition Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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- Definition Amount of decrease in unrecognized tax benefits resulting from lapses of applicable statutes of limitations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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