Income Taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended				12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Income Tax Contingency [Line Items]
Federal corporate statutory income tax blended rate					24.50%	35.00%	35.00%
Increase in income tax resulting from U.S. tax reform legislation					$ 71
Repatriation tax on unremitted non-U.S. earnings					4
Estimated incremental withholding taxes	$ 2				2	$ 3
Income tax expense (benefit)	2	$ 3	$ 2	$ 71	166	186	$ 148
Gross liability resulting from mandatory deemed repatriation	23				23
Previously unrecognized foreign tax credits					19
Amount of unrecognized tax benefit, if recognized, would affect the tax rate	10				10	10
KENTUCKY
Income Tax Contingency [Line Items]
Income tax expense (benefit)					4
Ashland | Tax Matters Agreement | Affiliated Entity
Income Tax Contingency [Line Items]
Increase (decrease) in due to affiliates recognized in selling, general and administrative expense					7	(16)
Tax benefit primarily related to higher expected utilization of tax attributes payable					3
Income tax expense (benefit)					(5)	16
Net tax-sharing payments						48
Due to (from) related party	$ (66)				(66)	$ (62)
Ashland | Tax Matters Agreement | Affiliated Entity | Legacy And Separation-Related Expenses, Net
Income Tax Contingency [Line Items]
Increase (decrease) in due to affiliates recognized in selling, general and administrative expense					8
Ashland | Tax Matters Agreement | Affiliated Entity | KENTUCKY
Income Tax Contingency [Line Items]
Increase (decrease) in due to affiliates recognized in selling, general and administrative expense					4
Income tax expense (benefit)					4
Domestic Tax Authority
Income Tax Contingency [Line Items]
Increase in income tax resulting from U.S. tax reform legislation					67
Accumulated other comprehensive (loss) income
Income Tax Contingency [Line Items]
Reclassification of income tax effects of U.S. tax reform					$ 8