Income Taxes - Components of Provision (Benefit) for Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Current
Current income tax expense (benefit)	$ 122	$ 136	$ 151
Deferred
Deferred income tax expense (benefit)	288	344	(52)
Income tax provision	410	480	99
Foreign tax authority | Ireland
Current
Current foreign tax expense	0	1	0
Deferred
Deferred foreign tax expense	0	0	(2)
Income tax provision	410	480	99
Foreign tax authority | Other Foreign
Current
Current foreign tax expense	118	128	137
Deferred
Deferred foreign tax expense	287	(73)	(63)
United States
Current
US - Federal and State	4	7	14
Deferred
US - Federal and State	$ 1	$ 417	$ 13