Revenue Recognition - Disaggregation of Revenue by Geographical Market (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue	$ 4,228	$ 4,596	$ 8,386	$ 8,800
Inter-segment elimination
Disaggregation of Revenue [Line Items]
Revenue	(64)	(91)	(131)	(174)
Americas | Operating segments
Disaggregation of Revenue [Line Items]
Revenue	1,915	1,941	3,850	3,727
Americas | Operating segments | United States
Disaggregation of Revenue [Line Items]
Revenue	1,630	1,634	3,245	3,112
Americas | Operating segments | Mexico
Disaggregation of Revenue [Line Items]
Revenue	643	656	1,313	1,280
Americas | Operating segments | Other Americas
Disaggregation of Revenue [Line Items]
Revenue	100	135	224	280
Americas | Regional elimination
Disaggregation of Revenue [Line Items]
Revenue	(458)	(484)	(932)	(945)
EMEA | Operating segments
Disaggregation of Revenue [Line Items]
Revenue	1,778	2,056	3,418	3,909
EMEA | Operating segments | Germany
Disaggregation of Revenue [Line Items]
Revenue	390	494	731	939
EMEA | Operating segments | Other EMEA
Disaggregation of Revenue [Line Items]
Revenue	1,904	2,112	3,650	3,988
EMEA | Regional elimination
Disaggregation of Revenue [Line Items]
Revenue	(516)	(550)	(963)	(1,018)
Asia | Operating segments
Disaggregation of Revenue [Line Items]
Revenue	599	690	1,249	1,338
Asia | Operating segments | China
Disaggregation of Revenue [Line Items]
Revenue	129	154	284	344
Asia | Operating segments | Other Asia
Disaggregation of Revenue [Line Items]
Revenue	471	536	967	994
Asia | Regional elimination
Disaggregation of Revenue [Line Items]
Revenue	$ (1)	$ 0	$ (2)	$ 0