Stockholders' Deficit - Narrative (Details) shares in Millions		12 Months Ended
	May 12, 2017 shares	Sep. 30, 2018 USD ($) shares	Sep. 30, 2017 shares	Sep. 30, 2016 shares	Jan. 31, 2018 USD ($)	Apr. 24, 2017 USD ($)
Class of Stock [Line Items]
Distribution of shares of common stock on a pro rata dividend on shares of Ashland Common stock outstanding (in shares) | shares		197	204	170
Affiliated Entity
Class of Stock [Line Items]
Number of shares issued per share of Ashland common stock owned	2.745338
Ashland | Affiliated Entity
Class of Stock [Line Items]
Distribution of shares of common stock on a pro rata dividend on shares of Ashland Common stock outstanding (in shares) | shares	170			170
Common stock
Class of Stock [Line Items]
Authorized shares for repurchase | $					$ 300,000,000	$ 150,000,000
Remaining authorized repurchase amount | $		$ 75,000,000