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Investments (Fair Value of our Marketable Debt Securities by Contractual Maturity) (Details)
$ in Millions
Sep. 30, 2017
USD ($)
Schedule of Held-to-maturity Securities [Line Items]  
Due in one year or less $ 183
Due after one year through five years 410
Due after five years through ten years 366
Due after ten years 624
Total 1,583
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 192
Due after five years through ten years 189
Due after ten years 316
Total $ 757