Significant Accounting Policies - Computation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Numerator
Income (loss) from continuing operations attributable to Ashland, net of tax	$ 168	$ 181	$ 173
Denominator
Denominator for basic EPS - Weighted-average common shares outstanding	53	55	60
Share based awards convertible to common shares	1	1	1
Denominator for diluted EPS - Adjusted weighted - average shares and assumed conversions	54	56	61
Earnings Per Share from Continuing Operations [Abstract]
Basic	$ 3.18	$ 3.26	$ 2.85
Diluted	$ 3.13	$ 3.2	$ 2.82