Income Taxes - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020	Sep. 28, 2016
Income Tax Contingency [Line Items]
Deferred tax assets, valuation allowance	$ 3.0	$ 33.3
Income Tax Benefit	29.0
Deferred Income Taxes and Tax Credits	25.7
Unrecognized tax benefits	35.7	8.2	$ 8.7	$ 13.4
Income tax expense	37.1	34.7	59.9
Other noncurrent liabilities	346.8	237.1
Tax Year 2023
Income Tax Contingency [Line Items]
Deferred tax assets, valuation allowance	27.5
Majority Shareholder
Income Tax Contingency [Line Items]
Ashland ownership percentage					83.00%
Tax Matters Agreement
Income Tax Contingency [Line Items]
Unrecognized tax benefits			33.0
Income before income taxes			26.8
Income tax expense			$ 5.8
Tax Matters Agreement | Ashland | Affiliated Entity
Income Tax Contingency [Line Items]
Other noncurrent liabilities	$ 10.8	$ 0.6