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Employee Benefit Plans (Net Periodic Benefit (Credit) Cost and Amounts Recognized in Other Comprehensive Income and Regulatory Assets and Liabilities) (Detail) - USD ($) $ in Millions |  |  |  | 12 Months Ended
--- | --- | --- | --- | ---
Sep. 30, 2017 | Jun. 30, 2017 | Jan. 31, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Expected return on plan assets |  |  |  |  | $ (767) | $ (691) |
Dominion Energy Gas Holdings, LLC |  |  |  |  |  |  |
Expected return on plan assets |  |  |  |  | (165) | (157) |
Pension Benefits |  |  |  |  |  |  |
Service cost |  |  |  |  | 138 | 118 | $ 126
Interest cost |  |  |  |  | 345 | 317 | 287
Expected return on plan assets |  |  |  |  | (639) | (573) | (531)
Amortization of prior service (credit) cost |  |  |  |  | 1 | 1 | 2
Amortization of net actuarial loss |  |  |  |  | 162 | 111 | 160
Settlements and curtailments |  |  |  |  |  | 1 |
Net periodic benefit (credit) cost |  |  |  |  | 7 | (25) | 44
Changes in plan assets and benefit obligations recognized in other comprehensive income and regulatory assets and liabilities: |  |  |  |  |  |  |
Current year net actuarial (gain) loss |  |  |  |  | 142 | 931 | 159
Prior service (credit) cost |  |  |  |  | 5 |  |
Settlements and curtailments |  |  |  |  | 1 | (1) |
Amortization of net actuarial loss |  |  |  |  | (162) | (111) | (160)
Amortization of prior service credit (cost) |  |  |  |  | (1) | (1) | (2)
Total recognized in other comprehensive income and regulatory assets and liabilities |  |  |  |  | $ (15) | $ 818 | $ (3)
Significant assumptions used to determine periodic cost: |  |  |  |  |  |  |
Discount rate |  | 4.05% | 4.05% | 4.51% |  | 4.46% | 4.40%
Expected long-term rate of return on plan assets |  |  |  |  | 8.75% | 8.75% | 8.75%
Weighted average rate of increase for compensation |  |  |  |  | 4.09% | 4.22% | 4.22%
Pension Benefits | Dominion Energy Gas Holdings, LLC |  |  |  |  |  |  |
Service cost |  |  |  |  | $ 15 | $ 13 | $ 15
Interest cost |  |  |  |  | 30 | 30 | 27
Expected return on plan assets |  |  |  |  | (141) | (134) | (126)
Amortization of prior service (credit) cost |  |  |  |  |  |  | 1
Amortization of net actuarial loss |  |  |  |  | 16 | 13 | 20
Net periodic benefit (credit) cost |  |  |  |  | (80) | (78) | (63)
Changes in plan assets and benefit obligations recognized in other comprehensive income and regulatory assets and liabilities: |  |  |  |  |  |  |
Current year net actuarial (gain) loss |  |  |  |  | (75) | 91 | 97
Amortization of net actuarial loss |  |  |  |  | (16) | (13) | (20)
Amortization of prior service credit (cost) |  |  |  |  |  |  | (1)
Total recognized in other comprehensive income and regulatory assets and liabilities |  |  |  |  | $ (91) | $ 78 | $ 76
Significant assumptions used to determine periodic cost: |  |  |  |  |  |  |
Discount rate |  |  |  |  | 4.50% | 4.99% | 4.40%
Expected long-term rate of return on plan assets |  |  |  |  | 8.75% | 8.75% | 8.75%
Weighted average rate of increase for compensation |  |  |  |  | 4.11% | 3.93% | 3.93%
Pension Benefits | Minimum |  |  |  |  |  |  |
Significant assumptions used to determine periodic cost: |  |  |  |  |  |  |
Discount rate |  |  |  |  | 3.31% | 2.87% |
Pension Benefits | Maximum |  |  |  |  |  |  |
Significant assumptions used to determine periodic cost: |  |  |  |  |  |  |
Discount rate |  |  |  |  | 4.50% | 4.99% |
Other Postretirement Benefits |  |  |  |  |  |  |
Service cost |  |  |  |  | $ 26 | $ 31 | $ 40
Interest cost |  |  |  |  | 60 | 65 | 67
Expected return on plan assets |  |  |  |  | (128) | (118) | (117)
Amortization of prior service (credit) cost |  |  |  |  | (51) | (35) | (27)
Amortization of net actuarial loss |  |  |  |  | 13 | 8 | 6
Net periodic benefit (credit) cost |  |  |  |  | (80) | (49) | (31)
Changes in plan assets and benefit obligations recognized in other comprehensive income and regulatory assets and liabilities: |  |  |  |  |  |  |
Current year net actuarial (gain) loss |  |  |  |  | 12 | 178 | (18)
Prior service (credit) cost |  |  |  |  | (73) | (216) | (31)
Settlements and curtailments |  |  |  |  | 2 |  |
Amortization of net actuarial loss |  |  |  |  | (13) | (8) | (6)
Amortization of prior service credit (cost) |  |  |  |  | 51 | 35 | 27
Total recognized in other comprehensive income and regulatory assets and liabilities |  |  |  |  | $ (21) | $ (11) | $ (28)
Significant assumptions used to determine periodic cost: |  |  |  |  |  |  |
Discount rate |  |  |  |  |  |  | 4.40%
Expected long-term rate of return on plan assets |  |  |  |  | 8.50% | 8.50% | 8.50%
Weighted average rate of increase for compensation |  |  |  |  | 3.29% | 4.22% | 4.22%
Healthcare cost trend rate |  |  |  |  | 7.00% | 7.00% | 7.00%
Rate to which the cost trend rate is assumed to decline (the ultimate trend rate) | [1] |  |  |  | 5.00% | 5.00% | 5.00%
Year that the rate reaches the ultimate trend rate | [1],[2] |  |  |  | 2021 | 2020 | 2019
Other Postretirement Benefits | Dominion Energy Gas Holdings, LLC |  |  |  |  |  |  |
Service cost |  |  |  |  | $ 4 | $ 5 | $ 7
Interest cost |  |  |  |  | 12 | 14 | 14
Expected return on plan assets |  |  |  |  | (24) | (23) | (24)
Amortization of prior service (credit) cost |  |  |  |  | (3) | 1 | (1)
Amortization of net actuarial loss |  |  |  |  | 2 | 1 | 2
Net periodic benefit (credit) cost |  |  |  |  | (9) | (2) | (2)
Changes in plan assets and benefit obligations recognized in other comprehensive income and regulatory assets and liabilities: |  |  |  |  |  |  |
Current year net actuarial (gain) loss |  |  |  |  | 18 | 28 | (9)
Prior service (credit) cost |  |  |  |  | (61) |  |
Amortization of net actuarial loss |  |  |  |  | (2) | (1) | (2)
Amortization of prior service credit (cost) |  |  |  |  | 3 | (1) | 1
Total recognized in other comprehensive income and regulatory assets and liabilities |  |  |  |  | $ (42) | $ 26 | $ (10)
Significant assumptions used to determine periodic cost: |  |  |  |  |  |  |
Discount rate |  |  |  |  | 4.47% | 4.93% | 4.40%
Expected long-term rate of return on plan assets |  |  |  |  | 8.50% | 8.50% | 8.50%
Weighted average rate of increase for compensation |  |  |  |  | 4.11% | 3.93% | 3.93%
Healthcare cost trend rate |  |  |  |  | 7.00% | 7.00% | 7.00%
Rate to which the cost trend rate is assumed to decline (the ultimate trend rate) | [1] |  |  |  | 5.00% | 5.00% | 5.00%
Year that the rate reaches the ultimate trend rate | [1] |  |  |  | 2021 | 2020 | 2019
Other Postretirement Benefits | Minimum |  |  |  |  |  |  |
Significant assumptions used to determine periodic cost: |  |  |  |  |  |  |
Discount rate |  |  |  |  | 3.92% | 3.56% |
Other Postretirement Benefits | Maximum |  |  |  |  |  |  |
Significant assumptions used to determine periodic cost: |  |  |  |  |  |  |
Discount rate |  |  |  |  | 4.47% | 4.94% |
[1] | Assumptions used to determine net periodic cost for the following year.
[2] | The Society of Actuaries model used to determine healthcare cost trend rates was updated in 2014. The new model converges to the ultimate trend rate much more quickly than previous models.

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- Definition Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, before Tax + References No definition available. + Details
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- Definition Other comprehensive income (loss), defined benefit plan, gain (loss) arising during period, before tax. + References No definition available. + Details
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- Definition Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), before Tax + References No definition available. + Details
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- Definition Other comprehensive income (loss), defined benefit plan, settlement or curtailment gain (loss), before tax. + References No definition available. + Details
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- Definition Amount of gain (loss) recognized in net periodic benefit (cost) credit of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(4) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of prior service cost (credit) recognized in net periodic benefit cost (credit) of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(5) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of expected return (loss) recognized in net periodic benefit (cost) credit, calculated based on expected long-term rate of return and market-related value of plan assets of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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