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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, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Expected return on plan assets |  |  |  |  | $ (806) | $ (767) |
Amortization of prior service (credit) cost |  |  |  |  | 1 | 1 | $ 1
Changes in plan assets and benefit obligations recognized in other comprehensive income and regulatory assets and liabilities: |  |  |  |  |  |  |
Amortization of prior service credit (cost) |  |  |  |  | (1) | (1) | (1)
Dominion Energy Gas Holdings, LLC |  |  |  |  |  |  |
Expected return on plan assets |  |  |  |  | (178) | (165) |
Pension Benefits |  |  |  |  |  |  |
Service cost |  |  |  |  | 157 | 138 | 118
Interest cost |  |  |  |  | 337 | 345 | 317
Expected return on plan assets |  |  |  |  | (663) | (639) | (573)
Amortization of prior service (credit) cost |  |  |  |  | 1 | 1 | 1
Amortization of net actuarial loss |  |  |  |  | 193 | 162 | 111
Settlements and curtailments |  |  |  |  |  |  | 1
Net periodic benefit (credit) cost |  |  |  |  | 25 | 7 | (25)
Changes in plan assets and benefit obligations recognized in other comprehensive income and regulatory assets and liabilities: |  |  |  |  |  |  |
Current year net actuarial (gain) loss |  |  |  |  | 490 | 142 | 931
Prior service (credit) cost |  |  |  |  |  | 5 |
Settlements and curtailments |  |  |  |  |  | 1 | (1)
Amortization of net actuarial loss |  |  |  |  | (193) | (162) | (111)
Amortization of prior service credit (cost) |  |  |  |  | (1) | (1) | (1)
Total recognized in other comprehensive income and regulatory assets and liabilities |  |  |  |  | $ 296 | $ (15) | $ 818
Significant assumptions used to determine periodic cost: |  |  |  |  |  |  |
Discount rate |  | 4.05% | 4.05% | 4.51% |  |  | 4.46%
Expected long-term rate of return on plan assets |  |  |  |  | 8.75% | 8.75% | 8.75%
Weighted average rate of increase for compensation |  |  |  |  | 4.09% | 4.09% | 4.22%
Pension Benefits | Dominion Energy Gas Holdings, LLC |  |  |  |  |  |  |
Service cost |  |  |  |  | $ 18 | $ 15 | $ 13
Interest cost |  |  |  |  | 29 | 30 | 30
Expected return on plan assets |  |  |  |  | (150) | (141) | (134)
Amortization of net actuarial loss |  |  |  |  | 19 | 16 | 13
Net periodic benefit (credit) cost |  |  |  |  | (84) | (80) | (78)
Changes in plan assets and benefit obligations recognized in other comprehensive income and regulatory assets and liabilities: |  |  |  |  |  |  |
Current year net actuarial (gain) loss |  |  |  |  | 207 | (75) | 91
Amortization of net actuarial loss |  |  |  |  | (19) | (16) | (13)
Total recognized in other comprehensive income and regulatory assets and liabilities |  |  |  |  | $ 188 | $ (91) | $ 78
Significant assumptions used to determine periodic cost: |  |  |  |  |  |  |
Discount rate |  |  |  |  | 3.81% | 4.50% | 4.99%
Expected long-term rate of return on plan assets |  |  |  |  | 8.75% | 8.75% | 8.75%
Weighted average rate of increase for compensation |  |  |  |  | 4.11% | 4.11% | 3.93%
Pension Benefits | Minimum |  |  |  |  |  |  |
Significant assumptions used to determine periodic cost: |  |  |  |  |  |  |
Discount rate |  |  |  |  | 3.80% | 3.31% | 2.87%
Pension Benefits | Maximum |  |  |  |  |  |  |
Significant assumptions used to determine periodic cost: |  |  |  |  |  |  |
Discount rate |  |  |  |  | 3.81% | 4.50% | 4.99%
Other Postretirement Benefits |  |  |  |  |  |  |
Service cost |  |  |  |  | $ 27 | $ 26 | $ 31
Interest cost |  |  |  |  | 56 | 60 | 65
Expected return on plan assets |  |  |  |  | (143) | (128) | (118)
Amortization of prior service (credit) cost |  |  |  |  | (52) | (51) | (35)
Amortization of net actuarial loss |  |  |  |  | 11 | 13 | 8
Net periodic benefit (credit) cost |  |  |  |  | (101) | (80) | (49)
Changes in plan assets and benefit obligations recognized in other comprehensive income and regulatory assets and liabilities: |  |  |  |  |  |  |
Current year net actuarial (gain) loss |  |  |  |  | 78 | 12 | 178
Prior service (credit) cost |  |  |  |  | (4) | (73) | (216)
Settlements and curtailments |  |  |  |  |  | 2 |
Amortization of net actuarial loss |  |  |  |  | (11) | (13) | (8)
Amortization of prior service credit (cost) |  |  |  |  | 52 | 51 | 35
Total recognized in other comprehensive income and regulatory assets and liabilities |  |  |  |  | $ 115 | $ (21) | $ (11)
Significant assumptions used to determine periodic cost: |  |  |  |  |  |  |
Discount rate |  |  |  |  | 3.76% |  |
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%
Healthcare cost trend rate | [1] |  |  |  | 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] |  |  |  | 2022 | 2021 | 2020
Other Postretirement Benefits | Dominion Energy Gas Holdings, LLC |  |  |  |  |  |  |
Service cost |  |  |  |  | $ 4 | $ 4 | $ 5
Interest cost |  |  |  |  | 11 | 12 | 14
Expected return on plan assets |  |  |  |  | (28) | (24) | (23)
Amortization of prior service (credit) cost |  |  |  |  | (4) | (3) | 1
Amortization of net actuarial loss |  |  |  |  | 3 | 2 | 1
Net periodic benefit (credit) cost |  |  |  |  | (14) | (9) | (2)
Changes in plan assets and benefit obligations recognized in other comprehensive income and regulatory assets and liabilities: |  |  |  |  |  |  |
Current year net actuarial (gain) loss |  |  |  |  | 16 | 18 | 28
Prior service (credit) cost |  |  |  |  | (4) | (61) |
Amortization of net actuarial loss |  |  |  |  | (3) | (2) | (1)
Amortization of prior service credit (cost) |  |  |  |  | 4 | 3 | (1)
Total recognized in other comprehensive income and regulatory assets and liabilities |  |  |  |  | $ 13 | $ (42) | $ 26
Significant assumptions used to determine periodic cost: |  |  |  |  |  |  |
Discount rate |  |  |  |  | 3.81% | 4.47% | 4.93%
Expected long-term rate of return on plan assets |  |  |  |  | 8.50% | 8.50% | 8.50%
Weighted average rate of increase for compensation |  |  |  |  | 4.55% | 4.11% | 3.93%
Healthcare cost trend rate | [1] |  |  |  | 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] |  |  |  | 2022 | 2021 | 2020
Other Postretirement Benefits | Minimum |  |  |  |  |  |  |
Significant assumptions used to determine periodic cost: |  |  |  |  |  |  |
Discount rate |  |  |  |  |  | 3.92% | 3.56%
Weighted average rate of increase for compensation |  |  |  |  | 3.95% |  |
Other Postretirement Benefits | Maximum |  |  |  |  |  |  |
Significant assumptions used to determine periodic cost: |  |  |  |  |  |  |
Discount rate |  |  |  |  |  | 4.47% | 4.94%
Weighted average rate of increase for compensation |  |  |  |  | 4.11% |  |
[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 Amount of gain (loss) recognized in net periodic benefit (cost) credit of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(4) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2709-114920 + 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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