Investments (Fair Value of our Marketable Debt Securities by Contractual Maturity) (Details) $ in Millions	Dec. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less	$ 192
Due after one year through five years	418
Due after five years through ten years	368
Due after ten years	568
Total	1,546
Virginia Electric and Power Company
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less	55
Due after one year through five years	181
Due after five years through ten years	208
Due after ten years	285
Total	$ 729