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Derivatives and Hedge Accounting Activities (Schedule of Offsetting Liabilities) (Parenthetical) (Detail) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Offsetting Liabilities [Line Items]    
Derivative liabilities, not subject to a master netting or similar arrangement $ 1 $ 1
Virginia Electric and Power Company    
Offsetting Liabilities [Line Items]    
Derivative liabilities, not subject to a master netting or similar arrangement $ 9 $ 5