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Income Taxes - Additional Information (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Income Tax Contingency [Line Items]        
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent 21.00% 24.50% 35.00% 35.00%
Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount   $ 2 $ 5  
One-time transition tax liability, unfavorable tax adjustment $ 29      
Increase (Decrease) in Deferred Income Taxes   (139)    
Increase (Decrease) in Deferred Liabilities   128    
Additional unfavorable adjustment 29      
Unrecognized tax benefits 165 164 [1] 194 [1]  
Total amount of unrecognized tax benefits that, if recognized, would affect the tax rate for continuing and discontinued operations 155      
Interest and penalties related to uncertain tax positions (5) 1 $ (3)  
Interest and penalties related to unrecognized tax benefits accrued 26 25    
Minimum [Member]        
Income Tax Contingency [Line Items]        
Decrease in Unrecognized Tax Benefits is Reasonably Possible 1      
Maximum [Member]        
Income Tax Contingency [Line Items]        
Decrease in Unrecognized Tax Benefits is Reasonably Possible $ 2      
Transition Tax [Member]        
Income Tax Contingency [Line Items]        
Provisional income tax expense benefit   128    
One-time transition tax liability, income tax expense benefit   187    
Deferred Rate [Member]        
Income Tax Contingency [Line Items]        
Provisional income tax expense benefit   139    
Other Tax Expense [Member]        
Income Tax Contingency [Line Items]        
Provisional income tax expense benefit   13    
Foreign Tax Authority [Member]        
Income Tax Contingency [Line Items]        
One-time transition tax liability, income tax expense benefit   46    
Uncertain Tax Position [Member]        
Income Tax Contingency [Line Items]        
One-time transition tax liability, income tax expense benefit   9    
State and Local Jurisdiction [Member]        
Income Tax Contingency [Line Items]        
One-time transition tax liability, income tax expense benefit   2    
Other Miscellaneous Items [Member]        
Income Tax Contingency [Line Items]        
One-time transition tax liability, income tax expense benefit   $ 6    
[1] Ashland has indemnity receivables from Valvoline and Pharmachem for $32 million and $39 million of the gross unrecognized tax benefits at September 30, 2019 and 2018, respectively.