Investments (Fair Value of our Marketable Debt Securities by Contractual Maturity) (Detail) $ in Millions	Sep. 30, 2018 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Due in one year or less	$ 199
Due after one year through five years	344
Due after five years through ten years	389
Due after ten years	657
Total	1,589
Virginia Electric and Power Company
Schedule of Held-to-maturity Securities [Line Items]
Due in one year or less	64
Due after one year through five years	139
Due after five years through ten years	221
Due after ten years	347
Total	$ 771