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Fair Value Measurements - ARO Trust (Details) - Transcontinental Gas Pipe Line Company, LLC [Member] - USD ($) $ in Millions | Sep. 30, 2025 | Dec. 31, 2024
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Trading, and Equity Securities, FV-NI, Cost | $ 219 | $ 200
Debt Securities, Trading, and Equity Securities, FV-NI | 346 | 297
External Trust |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Transco's annual funding commitment for ARO | 64 |
Cash management funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Trading, and Equity Securities, FV-NI, Cost | 30 | 27
Debt Securities, Trading, and Equity Securities, FV-NI | 30 | 27
Private Equity Funds, US |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Trading, and Equity Securities, FV-NI, Cost | 53 | 53
Debt Securities, Trading, and Equity Securities, FV-NI | 165 | 146
Private Equity Funds, Non-US |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Trading, and Equity Securities, FV-NI, Cost | 32 | 32
Debt Securities, Trading, and Equity Securities, FV-NI | 50 | 40
Municipal Bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Trading, and Equity Securities, FV-NI, Cost | 104 | 88
Debt Securities, Trading, and Equity Securities, FV-NI | $ 101 | $ 84

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