Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Numerator
Net income	$ 45	$ 64	$ 67	$ (10)	$ 105	$ 56	$ 71	$ 72	$ 166	$ 304	$ 273
Denominator
Weighted-average shares used to compute basic EPS (in shares)									197	204	170
Effect of dilutive securities (in shares)									0	0	0
Weighted-average shares used to compute diluted EPS (in shares)									197	204	170
Earnings per share
Earnings per share, basic (usd per share)	$ 0.23	$ 0.33	$ 0.33	$ (0.05)	$ 0.52	$ 0.27	$ 0.35	$ 0.35	$ 0.84	$ 1.49	$ 1.60
Earnings per share, diluted (usd per share)	$ 0.23	$ 0.33	$ 0.33	$ (0.05)	$ 0.52	$ 0.27	$ 0.35	$ 0.35	$ 0.84	$ 1.49	$ 1.60
Ashland | Majority Shareholder
Denominator
Weighted-average shares used to compute diluted EPS (in shares)											170