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Commitment and Contingencies (Schedule of Long Term Purchase Commitments) (Detail) - Virginia Electric and Power Company - Purchased electric capacity
$ in Millions
Dec. 31, 2017
USD ($)
 [1]
Long-term Purchase Commitment [Line Items]  
2018 $ 93
2019 61
2020 52
2021 46
2022 0
Thereafter 0
Total $ 252
[1] Commitments represent estimated amounts payable for capacity under a power purchase contract with a qualifying facility and an independent power producer, which ends in 2021. Capacity payments under the contract are generally based on fixed dollar amounts per month, subject to escalation using broad-based economic indices. At December 31, 2017, the present value of Virginia Power's total commitment for capacity payments is $221 million. Capacity payments totaled $114 million, $248 million, and $305 million, and energy payments totaled $72 million, $126 million, and $198 million for the years ended 2017, 2016 and 2015, respectively.