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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.

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