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Fair Value Measurements (Narrative) (Detail) - USD ($)
3 Months Ended 6 Months Ended
Mar. 31, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Unrealized gains or losses included in earnings in Level 3 fair value category $ 0 $ 0 $ 0 $ 0
Virginia Electric and Power Company        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Unrealized gains or losses included in earnings in Level 3 fair value category $ 0 $ 0 $ 0 $ 0