Restructuring and Impairment Costs - Changes in Reserve (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Mar. 31, 2021	Sep. 30, 2020
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	$ 237
Noncash adjustment		$ 20
Restructuring reserve ending balance	150	237
2021 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	13
Noncash adjustment	9
Restructuring reserve ending balance	13	13
2020 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	169
Utilized—cash	(60)
Noncash adjustment	(1)
Restructuring reserve ending balance	108	169
2019 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	35
Utilized—cash	(17)
Noncash adjustment	0
Restructuring reserve ending balance	18	35
Restructuring Plan 2018, 2017, 2016
Restructuring Reserve [Roll Forward]
Utilized—cash	(17)
Employee Severance and Termination Benefits | 2021 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	13
Restructuring reserve ending balance	13	13
Employee Severance and Termination Benefits | 2020 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	168
Utilized—cash	(60)
Noncash adjustment	0
Restructuring reserve ending balance	108	168
Employee Severance and Termination Benefits | 2019 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	32
Utilized—cash	(17)
Noncash adjustment	0
Restructuring reserve ending balance	15	32
Other | 2020 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	0
Utilized—cash	0
Noncash adjustment	(3)
Restructuring reserve ending balance	(3)	0
Other | 2019 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	3
Utilized—cash	0
Noncash adjustment	(1)
Restructuring reserve ending balance	2	3
Currency Translation | 2020 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	1
Utilized—cash	0
Noncash adjustment	2
Restructuring reserve ending balance	3	1
Currency Translation | 2019 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring reserve beginning balance	0
Utilized—cash	0
Noncash adjustment	1
Restructuring reserve ending balance	$ 1	$ 0