Litigation, Claims and Contingencies - Additional Information (Details) $ in Millions	3 Months Ended	6 Months Ended			12 Months Ended
	Jun. 30, 2019 USD ($)	Mar. 31, 2020 USD ($) WasteTreatmentorDisposalSite Facility ServiceStationProperty		Mar. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)		Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
Environmental Remediation Costs Recognized [Abstract]
Number of waste treatment or disposal sites were company is identified as a potentially responsible party under the superfund or similar state laws | WasteTreatmentorDisposalSite		79
Number of current and former operating facilities subject to various environmental laws | Facility		113
Total number of service station properties subject to various environmental laws | ServiceStationProperty		1,225
Number of service stations being actively remediated | ServiceStationProperty		28
Accrual for environmental loss contingencies		$ 178		$ 179	$ 186		$ 187
Accrued environmental loss contingencies, noncurrent		135			143
Recorded third-party environmental recoveries receivable		14			13
Recorded third-party environmental recoveries, noncurrent		$ 12			12
Maximum reserve for remediation reserve related to any one site (in hundredths)		15.00%
Minimum [Member]
Asbestos litigation cost projection [Abstract]
Number of Years Included in Asbestos Assumption		40 years
Maximum [Member]
Asbestos litigation cost projection [Abstract]
Number of Years Included in Asbestos Assumption		50 years
Environmental Remediation Costs Recognized [Abstract]
Environmental exit costs, reasonably possible additional loss		$ 425
Ashland [Member]
Asbestos claims [Roll Forward]
Increase (Decrease) in asbestos-related claims	$ 1	0		0	1		(8)	$ 36
Total reserves for asbestos claims		337	[1]	366	352	[1]	380	419	$ 415
Movement in Loss Contingency Receivable, Increase (Decrease) [Roll Forward]
Insurance receivable		109	[2]	136	123	[2]	140	155	151
Receivable adjustment	(5)	0		0	(5)		(5)	15
Asbestos litigation cost projection [Abstract]
Possible total future litigation defense and claim settlement costs		600
Hercules [Member]
Asbestos claims [Roll Forward]
Increase (Decrease) in asbestos-related claims	(10)	0		0	(10)		(19)	16
Total reserves for asbestos claims		242	[3]	272	252	[3]	282	323	321
Movement in Loss Contingency Receivable, Increase (Decrease) [Roll Forward]
Insurance receivable		49		54	49		54	68	$ 63
Receivable adjustment	$ (5)	0		$ 0	$ (5)		$ (14)	$ 5
Asbestos litigation cost projection [Abstract]
Possible total future litigation defense and claim settlement costs		$ 450

[1] Included $28 million classified in accrued expenses and other liabilities on the Condensed Consolidated Balance Sheets as of both March 31, 2020 and September 30, 2019.

[2] Included $13 million and $15 million classified in accounts receivable on the Condensed Consolidated Balance Sheets as of March 31, 2020 and September 30, 2019, respectively

[3] Included $21 million classified in accrued expenses and other liabilities on the Condensed Consolidated Balance Sheets as of both March 31, 2020 and September 30, 2019.