Other Financial Data (Other Operating Cash Flow) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Disclosure Text Block [Abstract]
Pension expense		$ 49	$ 127	$ 95
Stock compensation expense		216	110	145
Transition impact of Tax Act	$ (43)	(189)	0	0
Deferred income taxes and other		(22)	27	45
Total		$ 54	$ 264	$ 285