Restructuring and Impairment Costs - Changes in Reserve (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Jun. 30, 2021	Sep. 30, 2020
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	$ 237
Noncash adjustment—underspend/other		$ 20
Restructuring reserve ending balance	135	237
2021 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	24
Utilized—cash	(2)
Noncash adjustment—underspend/other	14
Restructuring reserve ending balance	22	24
2020 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	169
Utilized—cash	(77)
Noncash adjustment—underspend/other	0
Restructuring reserve ending balance	92	169
2019 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	35
Utilized—cash	(21)
Noncash adjustment—underspend/other	(2)
Restructuring reserve ending balance	12	35
Employee Severance and Termination Benefits | 2021 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	24
Utilized—cash	(2)
Restructuring reserve ending balance	22	24
Employee Severance and Termination Benefits | 2020 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	168
Utilized—cash	(77)
Noncash adjustment—underspend/other	(4)
Restructuring reserve ending balance	87	168
Employee Severance and Termination Benefits | 2019 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	32
Utilized—cash	(21)
Noncash adjustment—underspend/other	0
Restructuring reserve ending balance	11	32
Other | 2019 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	3
Utilized—cash	0
Noncash adjustment—underspend/other	(3)
Restructuring reserve ending balance	0	3
Currency Translation | 2020 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	1
Utilized—cash	0
Noncash adjustment—underspend/other	4
Restructuring reserve ending balance	5	1
Currency Translation | 2019 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	0
Utilized—cash	0
Noncash adjustment—underspend/other	1
Restructuring reserve ending balance	$ 1	$ 0