Investments (Fair Value of our Marketable Debt Securities by Contractual Maturity) (Detail) $ in Millions	Jun. 30, 2018 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Due in one year or less	$ 321
Due after one year through five years	327
Due after five years through ten years	353
Due after ten years	576
Total	1,577
Virginia Electric and Power Company
Schedule of Held-to-maturity Securities [Line Items]
Due in one year or less	49
Due after one year through five years	158
Due after five years through ten years	216
Due after ten years	328
Total	$ 751