Income Taxes Reconciliation of Beginning and Ending Amount of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Beginning balance	$ 2,451	$ 2,358	$ 2,161
Additions for tax positions related to the current year	128	433	435
Additions for tax positions of prior years	129	347	7
Reductions for tax positions of prior years	(27)	(88)	(201)
Settlements with taxing authorities	(54)	0	(19)
Statute closings and audit resolutions	(99)	(599)	(25)
Ending balance	$ 2,528	$ 2,451	$ 2,358