Investments (Fair Value of our Marketable Debt Securities by Contractual Maturity) (Details) $ in Millions	Mar. 31, 2016 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Due in one year or less	$ 211
Due after one year through five years	423
Due after five years through ten years	380
Due after ten years	549
Total	1,563
Virginia Electric and Power Company
Schedule of Held-to-maturity Securities [Line Items]
Due in one year or less	74
Due after one year through five years	191
Due after five years through ten years	196
Due after ten years	270
Total	$ 731