Investments (Fair Value of our Marketable Debt Securities by Contractual Maturity) (Detail) $ in Millions	Dec. 31, 2019 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Due in one year or less	$ 198
Due after one year through five years	412
Due after five years through ten years	390
Due after ten years	727
Total	1,727
Virginia Electric and Power Company
Schedule of Held-to-maturity Securities [Line Items]
Due in one year or less	91
Due after one year through five years	175
Due after five years through ten years	206
Due after ten years	381
Total	$ 853