Commitment and Contingencies (Schedule of Long Term Purchase Commitments) (Detail) - Virginia Electric and Power Company - Purchased electric capacity $ in Millions	Dec. 31, 2018 USD ($)	[1]
Long-term Purchase Commitment [Line Items]
2019	$ 60
2020	52
2021	46
Thereafter	0
Total	$ 158

[1] Commitments represent estimated amounts payable for capacity under power purchase contracts with independent power producers, which end in 2021. Capacity payments under the contracts are generally based on fixed dollar amounts per month, subject to escalation using broad based economic indices. At December 31, 2018, the present value of Virginia Power's total commitment for capacity payments is $142 million. Capacity payments totaled $50 million, $114 million and $248 million, and energy payments totaled $42 million, $72 million and $126 million for the years ended 2018, 2017 and 2016, respectively.