Investments (Fair Value of our Marketable Debt Securities by Contractual Maturity) (Details) $ in Millions	Sep. 30, 2016 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Due in one year or less	$ 199
Due after one year through five years	475
Due after five years through ten years	365
Due after ten years	567
Total	1,606
Virginia Electric and Power Company
Schedule of Held-to-maturity Securities [Line Items]
Due in one year or less	51
Due after one year through five years	228
Due after five years through ten years	202
Due after ten years	270
Total	$ 751