Equity Items - Components of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Equity [Abstract]
Unrealized translation gain (loss), before tax		$ (9)	$ 55	$ (40)	$ 58
Unrealized translation gain (loss), tax		0	0	0	0
Unrealized translation gain (loss), net of tax		(9)	55	(40)	58
Adjustment of unrecognized prior service cost (credit), before tax		0	0	(7)	0
Adjustment of unrecognized prior service cost (credit), tax		0	0	1	0
Adjustment of unrecognized prior service cost (credit), net of tax		0	0	(6)	0
Unrealized gain on investment, before taxes	[1]	0	(5)	0	6
Unrealized gain on investment, tax	[1]	0	1	0	(1)
Unrealized gain on investment, net of tax	[1]	0	(4)	0	5
Reclassification adjustment for gains included in net income, before tax		0	(3)	0	(4)
Reclassification adjustment for gains included in net income, tax		0	1	0	1
Reclassification adjustment for gains included in net income, net of tax		0	(2)	0	(3)
Total other comprehensive income (loss), before tax		(9)	47	(47)	60
Total other comprehensive income (loss), tax		0	2	1	0
Other comprehensive income (loss)		$ (9)	$ 49	$ (46)	$ 60

[1] Due to the adoption of new accounting guidance in fiscal 2019, unrealized gains and losses on Ashland’s equity securities are now recognized in net income rather than AOCI. See Notes A and E for more information