Shareholders' Equity - Changes in Certain Components of Shareholders' Equity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended				6 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2020	Mar. 31, 2019
Dividends per Common Share (USD per share)	$ 0.79	$ 0.79	$ 0.77	$ 0.77	$ 1.58	$ 1.54
Beginning balance		$ 21,081			$ 21,081
Net income	$ 183		$ 20		461	$ 619
Ending balance	20,951				20,951
Common Stock Issued at Par Value
Beginning balance	347	347	347	$ 347	347	347
Ending balance	347	347	347	347	347	347
Capital in Excess of Par Value
Beginning balance	16,320	16,270	16,174	16,179	16,270	16,179
Common stock issued for share-based compensation and other plans, net	(91)	(32)	(57)	(97)
Share-based compensation	58	82	60	92
Ending balance	16,288	16,320	16,177	16,174	16,288	16,177
Retained Earnings
Beginning balance	12,938	12,913	13,018	12,596	12,913	12,596
Net income		278	20	599
Common dividends	(215)	(215)	(208)	(207)
Preferred dividends	(38)	(38)	(38)	(38)
Common stock issued for share-based compensation and other plans, net			(1)
Effect of changes in accounting principle				68
Ending balance	12,868	12,938	12,792	13,018	12,868	12,792
Deferred Compensation
Beginning balance	24	23	24	22	23	22
Common stock issued for share-based compensation and other plans, net	(1)	1	(1)	2
Ending balance	23	24	23	24	23	23
Treasury Stock
Beginning balance	$ (6,228)	$ (6,190)	$ (6,235)	$ (6,243)	$ (6,190)	$ (6,243)
Beginning balance (shares)	(75,514)	(76,260)	(77,624)	(78,463)	(76,260)	(78,463)
Common stock issued for share-based compensation and other plans, net	$ 70	$ (38)	$ 42	$ 9
Common stock issued for share-based compensation and other plans, net (in shares)	573	758	618	851
Common stock held in trusts, net	30	(12)	50	(12)
Ending balance	$ (6,158)	$ (6,228)	$ (6,192)	$ (6,235)	$ (6,158)	$ (6,192)
Ending balance (shares)	(74,911)	(75,514)	(76,955)	(77,624)	(74,911)	(76,955)