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Commitment and Contingencies (Schedule of Long Term Purchase Commitments) (Parenthetical) (Detail) - Virginia Electric and Power Company - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Long-term Purchase Commitment [Line Items] |  |  |
Present value of total commitment for capacity payments | $ 221 |  |
Purchased electric capacity |  |  |
Long-term Purchase Commitment [Line Items] |  |  |
Payments | 114 | $ 248 | $ 305
Energy payments |  |  |
Long-term Purchase Commitment [Line Items] |  |  |
Payments | $ 72 | $ 126 | $ 198

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- Definition The amount purchased during the period under an unrecorded unconditional purchase obligation (for example, under the take-or-pay or throughput contract). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 440 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=77896938&loc=d3e25336-109308 + Details
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