Employee Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Pension Plans [Member]
Net Periodic Benefit Costs [Abstract]
Service cost	[1]	$ 9	$ 8	$ 21
Interest cost	[2]	10	8	94
Curtailment, settlement and other	[2]	0	0	(64)
Expected return on plan assets	[2]	(12)	(11)	(149)
Amortization of prior service credit	[2]	0	0	(1)
Actuarial (gain) loss	[2]	$ (12)	$ 0	$ 208
Defined Benefit Plan, Net Periodic Benefit (Cost) Credit, Immediate Recognition of Actuarial Gain (Loss), Statement of Income or Comprehensive Income [Extensible List]		ash:OtherNetPeriodicBenefitIncomeCostsMember	ash:OtherNetPeriodicBenefitIncomeCostsMember	ash:OtherNetPeriodicBenefitIncomeCostsMember
Total net periodic benefit cost (income)		$ (5)	$ 5	$ 109
Weighted-Average Plan Assumptions [Abstract]
Discount rate for service cost	[3]	2.51%	1.92%	3.95%
Discount rate for interest cost	[3]	2.52%	2.22%	3.30%
Rate of compensation increase	[3]	2.55%	2.80%	3.04%
Expected long-term rate of return on plan assets	[3]	3.33%	3.41%	6.71%
Other Postretirement Benefit Plans [Member]
Net Periodic Benefit Costs [Abstract]
Service cost	[1]	$ 1	$ 1	$ 1
Interest cost	[2]	1	2	3
Curtailment, settlement and other	[2]	0	0	(34)
Expected return on plan assets	[2]	0	0	0
Amortization of prior service credit	[2]	0	0	(11)
Actuarial (gain) loss	[2]	$ 0	$ 6	$ 32
Defined Benefit Plan, Net Periodic Benefit (Cost) Credit, Immediate Recognition of Actuarial Gain (Loss), Statement of Income or Comprehensive Income [Extensible List]		ash:OtherNetPeriodicBenefitIncomeCostsMember	ash:OtherNetPeriodicBenefitIncomeCostsMember	ash:OtherNetPeriodicBenefitIncomeCostsMember
Total net periodic benefit cost (income)		$ 2	$ 9	$ (9)
Weighted-Average Plan Assumptions [Abstract]
Discount rate for service cost	[3]	3.93%	3.93%	4.07%
Discount rate for interest cost	[3]	3.13%	2.86%	2.57%

[1] Service cost was not impacted by new accounting guidance adopted in 2018 and is therefore still classified within the selling, general and administrative expense and cost of sales captions on the Statements of Consolidated Comprehensive Income (Loss). See Note A for additional information.

[2] These components are now classified within the other net periodic benefit income (costs) caption on the Statements of Consolidated Comprehensive Income (Loss) due to the adoption of new accounting guidance in 2018. See Note A for additional information.

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