Income Taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Tax Contingency [Line Items]
Income tax expense	$ 27	$ 38	$ 121	$ 76
Effective tax rate	28.70%	34.90%	68.00%	34.70%
Tax cuts and jobs act of 2017, income tax expense (benefit)			$ 70
US corporate federal blended income tax rate			24.50%
Increase in income tax resulting from U.S. tax reform legislation			$ 67
Repatriation tax on unremitted non-U.S. earnings			4
Estimated incremental withholding taxes	$ 2		2
Net deferred tax assets						$ 281
Gross liability	22		22
Foreign tax credits			18
Liability for uncertain income tax positions	10		10
Tax Matters Agreement | Ashland
Income Tax Contingency [Line Items]
Due to (from) related party	82		82			74
Tax Matters Agreement | Ashland | Accrued expenses and other liabilities
Income Tax Contingency [Line Items]
Due to (from) related party	3		3			2
Ashland | Tax Matters Agreement | Ashland
Income Tax Contingency [Line Items]
Income tax expense			3
Tax cuts and jobs act of 2017, income tax expense (benefit)			(3)
Increase in due to affiliates recognized in selling, general and administrative expense			7
Due to (from) related party	69		69			62
Ashland | Tax Matters Agreement | Ashland | Accrued expenses and other liabilities
Income Tax Contingency [Line Items]
Due to (from) related party	3		3			1
Ashland | Tax Matters Agreement | Ashland | Other noncurrent liabilities
Income Tax Contingency [Line Items]
Due to (from) related party	$ 66		66			$ 61
Scenario, Forecast
Income Tax Contingency [Line Items]
Uncertain tax positions from decreases in positions taken in the next twelve months					$ 3
Legacy and Separation-related Expenses, net | Ashland | Tax Matters Agreement | Ashland
Income Tax Contingency [Line Items]
Increase in due to affiliates recognized in selling, general and administrative expense			$ 8