Investments (Fair Value of our Marketable Debt Securities by Contractual Maturity) (Details) $ in Millions	Mar. 31, 2017 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Due in one year or less	$ 164
Due after one year through five years	434
Due after five years through ten years	385
Due after ten years	576
Total	1,559
Virginia Electric and Power Company
Schedule of Held-to-maturity Securities [Line Items]
Due in one year or less	60
Due after one year through five years	188
Due after five years through ten years	205
Due after ten years	286
Total	$ 739