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, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2021	Mar. 31, 2020
Beginning balance		$ 23,765			$ 23,765
Net income	$ 299		$ 183		1,302	$ 461
Ending balance	$ 24,826				$ 24,826
Dividends per Common Share (USD per share)	$ 0.83	$ 0.83	$ 0.79	$ 0.79	$ 1.66	$ 1.58
Common Stock  Issued at Par Value
Beginning balance	$ 365	$ 365	$ 347	$ 347	$ 365	$ 347
Ending balance	365	365	347	347	365	347
Capital in Excess of Par Value
Beginning balance	19,301	19,270	16,320	16,270	19,270	16,270
Common stock issued for share-based compensation and other plans, net	(15)	(53)	(91)	(32)
Share-based compensation	55	83	58	82
Ending balance	19,341	19,301	16,288	16,320	19,341	16,288
Retained Earnings
Beginning balance	13,522	12,791	12,938	12,913	12,791	12,913
Net income	299	1,003	183	278
Common dividends	(242)	(242)	(215)	(215)
Preferred dividends	(23)	(23)	(38)	(38)
Ending balance	13,557	13,522	12,868	12,938	13,557	12,868
Retained Earnings | Cumulative Effect, Period of Adoption, Adjustment
Stockholders' Equity, Other		(9)
Deferred Compensation
Beginning balance	23	23	24	23	23	23
Common stock issued for share-based compensation and other plans, net			(1)	1
Ending balance	23	23	23	24	23	23
Treasury Stock
Beginning balance	$ (6,136)	$ (6,138)	$ (6,228)	$ (6,190)	$ (6,138)	$ (6,190)
Beginning balance (shares)	(74,080)	(74,623)	(75,514)	(76,260)	(74,623)	(76,260)
Common stock issued for share-based compensation and other plans, net	$ 4	$ 2	$ 70	$ (38)
Common stock issued for share-based compensation and other plans, net (in shares)	234	549	573	758
Common stock held in trusts, net (in shares)	23	(7)	30	(12)
Ending balance	$ (6,132)	$ (6,136)	$ (6,158)	$ (6,228)	$ (6,132)	$ (6,158)
Ending balance (shares)	(73,821)	(74,080)	(74,911)	(75,514)	(73,821)	(74,911)