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Investments (Fair Value of our Marketable Debt Securities by Contractual Maturity) (Detail) $ in Millions | Mar. 31, 2018 USD ($)
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Schedule of Held-to-maturity Securities [Line Items] |
Due in one year or less | $ 165
Due after one year through five years | 375
Due after five years through ten years | 375
Due after ten years | 652
Total | 1,567
Virginia Electric and Power Company |
Schedule of Held-to-maturity Securities [Line Items] |
Due in one year or less | 43
Due after one year through five years | 155
Due after five years through ten years | 208
Due after ten years | 339
Total | $ 745

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