Investments (Fair Value of our Marketable Debt Securities by Contractual Maturity) (Detail) $ in Millions	Mar. 31, 2018 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Due in one year or less	$ 165
Due after one year through five years	375
Due after five years through ten years	375
Due after ten years	652
Total	1,567
Virginia Electric and Power Company
Schedule of Held-to-maturity Securities [Line Items]
Due in one year or less	43
Due after one year through five years	155
Due after five years through ten years	208
Due after ten years	339
Total	$ 745