Income Taxes - Effective Tax Rate Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Income Tax Examination [Line Items]
Goodwill Impairment	$ 2
Change in valuation allowance			$ 100
Income tax provision		$ 410	480	$ 99
Foreign tax authority | Ireland
Income Tax Examination [Line Items]
Tax expense at Ireland statutory rate		0	(140)	133
State and local income taxes, net of federal benefit		(41)	(60)	(10)
Foreign tax rate differential		(109)	(146)	(67)
Notional interest deduction		(63)	(66)	(28)
Credits and incentives		(9)	(13)	(13)
Gain on previously-held interest		0	0	(19)
Goodwill Impairment		0	(21)	0
Repatriation of foreign earnings		31	36	30
Foreign exchange		2	3	(11)
Impact of enacted tax rate changes		(5)	23	10
Impact of U.S. tax reform		0	210	0
Change in uncertain tax positions		107	97	50
Change in valuation allowance		503	554	21
Other		(6)	3	3
Income tax provision		$ 410	$ 480	$ 99