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Investments (Fair Value of our Marketable Debt Securities by Contractual Maturity) (Detail) $ in Millions | Jun. 30, 2018 USD ($)
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Schedule of Held-to-maturity Securities [Line Items] |
Due in one year or less | $ 321
Due after one year through five years | 327
Due after five years through ten years | 353
Due after ten years | 576
Total | 1,577
Virginia Electric and Power Company |
Schedule of Held-to-maturity Securities [Line Items] |
Due in one year or less | 49
Due after one year through five years | 158
Due after five years through ten years | 216
Due after ten years | 328
Total | $ 751

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