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Equity (Schedule of Changes in AOCI by Component Net of Tax) (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Beginning balance |  | $ 14,605 |  |
Net current period other comprehensive income (loss) |  | 141 | $ (325) | $ (58)
Ending balance |  | 17,142 | 14,605 |
Virginia Electric and Power Company |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Beginning balance |  | 11,865 |  |
Net current period other comprehensive income (loss) |  | 16 | 6 | (10)
Ending balance |  | 12,224 | 11,865 |
Dominion Energy Gas Holdings, LLC |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Net current period other comprehensive income (loss) |  | 25 | (24) | (13)
Deferred Gains and Losses on Derivatives-Hedging Activities |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Beginning balance |  | (280) | (176) |
Other comprehensive income before reclassifications: gains (losses) |  | 8 | 55 |
Amounts reclassified from AOCI: (gains) losses | [1] | (29) | (159) |
Net current period other comprehensive income (loss) |  | (21) | (104) |
Less other comprehensive income attributable to noncontrolling interest |  | 1 |  |
Ending balance |  | (302) | (280) | (176)
Deferred Gains and Losses on Derivatives-Hedging Activities | Virginia Electric and Power Company |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Beginning balance |  | (8) | (7) |
Other comprehensive income before reclassifications: gains (losses) |  | (5) | (2) |
Amounts reclassified from AOCI: (gains) losses | [1] | 1 | 1 |
Net current period other comprehensive income (loss) |  | (4) | (1) |
Ending balance |  | (12) | (8) | (7)
Deferred Gains and Losses on Derivatives-Hedging Activities | Dominion Energy Gas Holdings, LLC |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Beginning balance |  | (24) | (17) |
Other comprehensive income before reclassifications: gains (losses) |  | 5 | (16) |
Amounts reclassified from AOCI: (gains) losses | [1] | (4) | 9 |
Net current period other comprehensive income (loss) |  | 1 | (7) |
Ending balance |  | (23) | (24) | (17)
Unrealized Gains and Losses on Investment Securities |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Beginning balance |  | 569 | 504 |
Other comprehensive income before reclassifications: gains (losses) |  | 215 | 93 |
Amounts reclassified from AOCI: (gains) losses | [1] | (37) | (28) |
Net current period other comprehensive income (loss) |  | 178 | 65 |
Ending balance |  | 747 | 569 | 504
Unrealized Gains and Losses on Investment Securities | Virginia Electric and Power Company |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Beginning balance |  | 54 | 47 |
Other comprehensive income before reclassifications: gains (losses) |  | 24 | 11 |
Amounts reclassified from AOCI: (gains) losses | [1] | (4) | (4) |
Net current period other comprehensive income (loss) |  | 20 | 7 |
Ending balance |  | 74 | 54 | 47
Unrecognized Pension and Other Postretirement Benefit Costs |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Beginning balance |  | (1,082) | (797) |
Other comprehensive income before reclassifications: gains (losses) |  | (69) | (319) |
Amounts reclassified from AOCI: (gains) losses | [1] | 50 | 34 |
Net current period other comprehensive income (loss) |  | (19) | (285) |
Ending balance |  | (1,101) | (1,082) | (797)
Unrecognized Pension and Other Postretirement Benefit Costs | Dominion Energy Gas Holdings, LLC |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Beginning balance |  | (99) | (82) |
Other comprehensive income before reclassifications: gains (losses) |  | 20 | (20) |
Amounts reclassified from AOCI: (gains) losses | [1] | 4 | 3 |
Net current period other comprehensive income (loss) |  | 24 | (17) |
Ending balance |  | (75) | (99) | (82)
Other Comprehensive Loss From Equity Method Investee |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Beginning balance |  | (6) | (5) |
Other comprehensive income before reclassifications: gains (losses) |  | 3 | (1) |
Net current period other comprehensive income (loss) |  | 3 | (1) |
Ending balance |  | (3) | (6) | (5)
Accumulated Other Comprehensive Income (Loss) |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Beginning balance |  | (799) | (474) |
Other comprehensive income before reclassifications: gains (losses) |  | 157 | (172) |
Amounts reclassified from AOCI: (gains) losses | [1] | (16) | (153) |
Net current period other comprehensive income (loss) |  | 140 | (325) | (58)
Less other comprehensive income attributable to noncontrolling interest |  | 1 |  |
Ending balance |  | (659) | (799) | (474)
Accumulated Other Comprehensive Income (Loss) | Virginia Electric and Power Company |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Beginning balance |  | 46 | 40 |
Other comprehensive income before reclassifications: gains (losses) |  | 19 | 9 |
Amounts reclassified from AOCI: (gains) losses | [1] | (3) | (3) |
Net current period other comprehensive income (loss) |  | 16 | 6 | (10)
Ending balance |  | 62 | 46 | 40
Accumulated Other Comprehensive Income (Loss) | Dominion Energy Gas Holdings, LLC |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Beginning balance |  | (123) | (99) |
Other comprehensive income before reclassifications: gains (losses) |  | 25 | (36) |
Amounts reclassified from AOCI: (gains) losses | [1] | 0 | 12 |
Net current period other comprehensive income (loss) |  | 25 | (24) | (13)
Ending balance |  | $ (98) | $ (123) | $ (99)
[1] | See table below for details about these reclassifications.

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