Restructuring and Impairment Costs - Changes in Reserve (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Mar. 31, 2019	Sep. 30, 2018
Restructuring Reserve [Roll Forward]
Restructuring Reserve Beginning Balance	$ 141
Noncash adjustment—underspend		$ (7)
Restructuring Reserve Ending Balance	140	141
2019 Restructuring Plan
Restructuring Reserve [Roll Forward]
Noncash adjustment—underspend	8
2018 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring Reserve Beginning Balance	48
Utilized—cash	(16)
Utilized—noncash	0
Noncash adjustment—underspend	2
Restructuring Reserve Ending Balance	30	48
2016 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring Reserve Beginning Balance	75
Utilized—cash	(37)
Utilized—noncash	(2)
Noncash adjustment—underspend		(20)
Restructuring Reserve Ending Balance	36	75
Employee Severance and Termination Benefits | 2019 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring Reserve Beginning Balance	80
Utilized—cash	(13)
Restructuring Reserve Ending Balance	67	80
Employee Severance and Termination Benefits | 2018 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring Reserve Beginning Balance	49
Utilized—cash	(16)
Utilized—noncash	0
Noncash adjustment—underspend	2
Restructuring Reserve Ending Balance	31	49
Employee Severance and Termination Benefits | 2017 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring Reserve Beginning Balance	12
Utilized—cash	(2)
Utilized—noncash	(1)
Noncash adjustment—underspend		(6)
Restructuring Reserve Ending Balance	3	12
Employee Severance and Termination Benefits | 2016 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring Reserve Beginning Balance	71
Utilized—cash	(37)
Utilized—noncash	0
Noncash adjustment—underspend		(20)
Restructuring Reserve Ending Balance	34	71
Other | 2018 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring Reserve Beginning Balance	1
Utilized—cash	0
Utilized—noncash	(1)
Noncash adjustment—underspend	0
Restructuring Reserve Ending Balance	0	1
Currency Translation | 2018 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring Reserve Beginning Balance	(2)
Utilized—cash	0
Utilized—noncash	1
Noncash adjustment—underspend	0
Restructuring Reserve Ending Balance	(1)	(2)
Currency Translation | 2016 Restructuring Plan
Restructuring Reserve [Roll Forward]
Restructuring Reserve Beginning Balance	4
Utilized—cash	0
Utilized—noncash	(2)
Restructuring Reserve Ending Balance	$ 2	$ 4