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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
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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