Employee Benefit Plans - Pension and Other Postretirement Benefit Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Ashland
Weighted-average plan assumptions
Non-service income			$ 10
Ashland | Ashland
Net periodic benefit income
Service cost			7
Actuarial loss (gain)			24
Pension benefits
Net periodic benefit income
Service cost	$ 2	$ 2	3
Interest cost	75	86	11
Expected return on plan assets	(103)	(145)	(17)
Amortization of prior service credit	0	0	0
Actuarial loss (gain)	38	(63)	(42)
Pre-separation allocation from Ashland	0	0	21
Net periodic benefit (income) costs	$ 12	$ (120)	$ (24)
Weighted-average plan assumptions
Discount rate for service cost	2.94%	2.15%	4.10%
Discount rate for interest cost	3.23%	2.84%	3.23%
Rate of compensation increase	3.05%	2.99%	3.23%
Expected long-term rate of return on plan assets	5.17%	6.56%	6.77%
Pension benefits | UNITED STATES | Geographic Concentration Risk | Net Periodic Benefit Obligation
Weighted-average plan assumptions
Concentration risk	97.00%
Other postretirement benefits
Net periodic benefit income
Service cost	$ 0	$ 0	$ 0
Interest cost	2	1	0
Expected return on plan assets	0	0	0
Amortization of prior service credit	(12)	(12)	(1)
Actuarial loss (gain)	0	(5)	0
Pre-separation allocation from Ashland	0	0	0
Net periodic benefit (income) costs	$ (10)	$ (16)	$ (1)
Weighted-average plan assumptions
Discount rate for service cost	4.05%	2.95%	4.25%
Discount rate for interest cost	3.11%	2.64%	2.92%
Rate of compensation increase	0.00%	0.00%	0.00%
Expected long-term rate of return on plan assets	0.00%	0.00%	0.00%
Other postretirement benefits | UNITED STATES | Geographic Concentration Risk | Net Periodic Benefit Obligation
Weighted-average plan assumptions
Concentration risk	75.00%