Fair Value Measurements - Summary of Restricted Investment Portfolio (Details) - USD ($) $ in Millions	Jun. 30, 2025	Sep. 30, 2024
Restricted Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost	$ 324	$ 340
Gross Unrealized Gain	61	56
Gross Unrealized Loss	(35)	(28)
Fair Value	350	368
Fixed Income Mutual Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost	209	211
Gross Unrealized Gain	0	0
Gross Unrealized Loss	(35)	(28)
Fair Value	174	183
Equity Mutual Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost	113	124
Gross Unrealized Gain	61	56
Gross Unrealized Loss	0	0
Fair Value	174	180
Demand Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost	2	5
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Fair Value	$ 2	$ 5

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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+ Details

ASHLAND INC. 10-Q