Income Taxes Significant Components of Company's Income Tax Provision from Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Components of Income Tax [Line Items]
Tax expense at Ireland statutory rate		$ 132	$ 193	$ 144
U.S. state income tax, net of federal benefit		15	15	8
Income subject to the U.S. federal tax rate		(110)	39	(311)
Income subject to rates different than the statutory rate		38	(201)	185
Reserve and valuation allowance adjustments	$ (76)	(284)	31	(164)
Impact of acquisitions and divestitures		0	16	475
U.S. Tax Reform discrete items		0	108	0
Restructuring and impairment costs		(24)	(4)	(15)
Income tax provision		$ (233)	$ 197	$ 322
Ireland statutory rate		12.50%	12.50%	12.50%
IRELAND
Components of Income Tax [Line Items]
Ireland statutory rate		12.50%	12.50%	12.50%