Fair Value Measurements (Narrative) (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
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
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
Maximum
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	$ 1,000,000