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Employee Benefit Plans (Benefit Payments Expected Future Service) (Detail)
$ in Millions
Dec. 31, 2017
USD ($)
Pension Benefits  
Defined Benefit Plan Disclosure [Line Items]  
2018 $ 373
2019 378
2020 402
2021 418
2022 434
2023-2027 2,437
Pension Benefits | Dominion Energy Gas Holdings, LLC  
Defined Benefit Plan Disclosure [Line Items]  
2018 35
2019 37
2020 38
2021 39
2022 41
2023-2027 214
Other Postretirement Benefits  
Defined Benefit Plan Disclosure [Line Items]  
2018 99
2019 101
2020 102
2021 102
2022 102
2023-2027 486
Other Postretirement Benefits | Dominion Energy Gas Holdings, LLC  
Defined Benefit Plan Disclosure [Line Items]  
2018 19
2019 19
2020 20
2021 20
2022 20
2023-2027 $ 94