Employee Benefit Plans - Additional Information (Details) - USD ($)		12 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Pre-65 health care cost increase trend rate		6.60%
Future Pre-65 health care cost trend rate		4.50%
Prior service credit recognized in accumulated other comprehensive income		$ 0	$ 0
Defined Benefit Plan, Plan Assets Allocation [Abstract]
Historical investment strategy asset mix, fixed income (in hundredths)		49.00%
Historical investment strategy asset mix, risk assets (in hundredths)		38.00%
Historical investment strategy asset mix, other securities assets (in hundredths)		13.00%
United States Pension Plans of US Entity, Defined Benefit [Member]
Defined Benefit Plan, Plan Assets Allocation [Abstract]
Actual contributions to benefit plans in period		$ 1,000,000	1,000,000
Estimated future contributions in current next fiscal year		1,000,000
Foreign Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Plan Assets Allocation [Abstract]
Actual contributions to benefit plans in period		4,000,000	8,000,000
Estimated future contributions in current next fiscal year		5,000,000
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expects to recognize of prior service credit in accumulated other comprehensive income as next fiscal year		1,000,000
INEOS Enterprises [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, actuarial gain (loss)		27,000,000
Employee benefit obligations		27,000,000
Other Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service credit recognized in accumulated other comprehensive income		0	0
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service credit recognized in accumulated other comprehensive income		3,000,000	(4,000,000)
Weighted-Average Plan Assumptions [Abstract]
Defined benefit plan, accumulated benefit obligation		$ 400,000,000	$ 394,000,000
Expected long-term rate of return on plan assets	[1]	3.44%	3.36%	3.44%
U.S. Benefit Plans [Member]
Weighted-Average Plan Assumptions [Abstract]
Expected long-term rate of return on plan assets		3.44%	3.36%
Other Pension Plan, Postretirement or Supplemental Plans [Member]
Defined Benefit Plan, Expected Future Benefit Payments, Fiscal Year Maturity [Abstract]
Company contributions to other employee benefit savings plans		$ 26,000,000	$ 25,000,000	$ 26,000,000
Postemployment benefits liability, noncurrent		3,000,000	$ 7,000,000
Amendments to Plan [Member] | Other Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, actuarial gain (loss)				2,000,000
Employee benefit obligations				$ 2,000,000
Amendments to Plan [Member] | Foreign Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Remeasurement due to curtailment gain		$ 18,000,000

[1] The plan assumptions discussed are a blended weighted-average rate for Ashland’s U.S. and non-U.S. plans.