Restructuring and Impairment Costs - Changes in Reserve (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	$ 141
Restructuring reserve ending balance	123	$ 141
2019 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	105
Utilized—cash	(32)
Utilized—noncash	(3)
Noncash adjustment - underspend	(16)
Restructuring reserve ending balance	70	105
2018 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	48	71
Utilized—cash	(24)	(19)
Utilized—noncash	(3)	(4)
Noncash adjustment - underspend	(10)
Reserve adjustment	9
Restructuring reserve ending balance	20	48	$ 71
2017 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	12	38	46
Utilized—cash	(3)	(26)	(8)
Noncash adjustment - underspend	(4)
Restructuring reserve ending balance	5	12	38	$ 46
2016 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	75	160	213	332
Utilized—cash	(45)	(64)	(60)	(30)
Utilized—noncash	(2)	(1)	7	(89)
Noncash adjustment - underspend	(1)	(20)
Restructuring reserve ending balance	27	75	160	213
Employee Severance and Termination Benefits | 2019 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	101
Utilized—cash	(32)
Utilized—noncash	0
Restructuring reserve ending balance	69	101
Employee Severance and Termination Benefits | 2018 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	49	68
Utilized—cash	(24)	(19)
Utilized—noncash	0	0
Noncash adjustment - underspend	(10)
Reserve adjustment	9
Restructuring reserve ending balance	24	49	68
Employee Severance and Termination Benefits | 2017 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	12	38	42
Utilized—cash	(3)	(26)	(4)
Noncash adjustment - underspend	(4)
Restructuring reserve ending balance	5	12	38	42
Employee Severance and Termination Benefits | 2016 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	71	146	194	223
Utilized—cash	(45)	(55)	(48)	(29)
Utilized—noncash	0	0	0	0
Noncash adjustment - underspend	(1)	(20)
Restructuring reserve ending balance	25	71	146	194
Long-Lived Asset Impairments | 2016 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	0	0	0	87
Utilized—cash	0	0	0	0
Utilized—noncash	0	0	0	(87)
Noncash adjustment - underspend	0	0
Restructuring reserve ending balance	0	0	0	0
Other | 2019 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	4
Utilized—cash	0
Utilized—noncash	(1)
Restructuring reserve ending balance	3	4
Other | 2018 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	1	3
Utilized—cash	0	0
Utilized—noncash	(1)	(2)
Noncash adjustment - underspend	0
Reserve adjustment	0
Restructuring reserve ending balance	0	1	3
Other | 2017 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	0	0	4
Utilized—cash	0	0	(4)
Noncash adjustment - underspend	0
Restructuring reserve ending balance	0	0	0	4
Other | 2016 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	0	9	21	22
Utilized—cash	0	(9)	(12)	(1)
Utilized—noncash	0	0	0	0
Noncash adjustment - underspend	0	0
Restructuring reserve ending balance	0	0	9	21
Currency Translation | 2019 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	0
Utilized—cash	0
Utilized—noncash	(2)
Restructuring reserve ending balance	(2)	0
Currency Translation | 2018 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	(2)	0
Utilized—cash	0	0
Utilized—noncash	(2)	(2)
Noncash adjustment - underspend	0
Reserve adjustment	0
Restructuring reserve ending balance	(4)	(2)	0
Currency Translation | 2016 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	4	5	(2)	0
Utilized—cash	0	0	0	0
Utilized—noncash	(2)	(1)	7	(2)
Noncash adjustment - underspend	0	0
Restructuring reserve ending balance	$ 2	$ 4	$ 5	$ (2)