Fair Value Measurements - Summary of Financial Instruments Subject to Recurring Fair Value Measurements (Parenthetical) (Details) - USD ($) $ in Millions	Jun. 30, 2026	Sep. 30, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted Investments	$ 285	$ 297
Restricted Investments, Current	47	50
Asbestos Trust [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value	220	231
Environmental Trust [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value	$ 112	$ 116

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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+ Details

ASHLAND INC. 10-Q