Earnings Per Share - Computation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended
		Dec. 31, 2019	Dec. 31, 2018
Numerator
Numerator for basic and diluted EPS - Income (loss) from continuing operations		$ 34	$ (71)
Denominator
Denominator for basic EPS - Weighted-average common shares outstanding		60	63
Share based awards convertible to common shares	[1]	1	0
Denominator for diluted EPS - Adjusted weighted-average shares and assumed conversions		61	63
PER SHARE DATA
Basic		$ 0.57	$ (1.14)
Diluted		$ 0.56	$ (1.14)

[1] As a result of the loss from continuing operations attributable to Ashland during the three months ended December 31, 2018, the effect of the share-based awards convertible to common shares would be antidilutive. In accordance with U.S. GAAP, they have been excluded from the diluted EPS calculation.