Investments (Fair Value of our Marketable Debt Securities by Contractual Maturity) (Details) $ in Millions	Jun. 30, 2017 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Due in one year or less	$ 149
Due after one year through five years	448
Due after five years through ten years	362
Due after ten years	599
Total	1,558
Virginia Electric and Power Company
Schedule of Held-to-maturity Securities [Line Items]
Due in one year or less	48
Due after one year through five years	209
Due after five years through ten years	182
Due after ten years	295
Total	$ 734