Shareholders' Equity - Changes in Certain Components of Shareholders' Equity (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance				$ 21,081					$ 21,081
Beginning balance (shares)				(76,259,835)					(76,259,835)
Net Income	$ 128	$ 286	$ 183	$ 278	$ 163	$ 451	$ 20	$ 599	$ 874	$ 1,233	$ 311
Ending balance	$ 23,765				$ 21,081				$ 23,765	$ 21,081
Ending balance (shares)	(74,622,657)				(76,259,835)				(74,622,657)	(76,259,835)
Common stock dividend per share (USD per share)									$ 3.16	$ 3.08	$ 3.00
Common Stock  Issued at Par Value
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance				347				347	$ 347	$ 347	$ 347
Ending balance	$ 365				$ 347				365	347	347
Stock Issued During Period, Value, Conversion of Convertible Securities, Net of Adjustments									12
Stock Issued During Period, Value, New Issues									6
Capital in Excess of Par Value
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance				16,270				16,179	16,270	16,179	9,619
Share-based compensation and other plans, net									(143)	(170)	(246)
Share-based compensation									244	261	328
Ending balance	19,270				16,270				19,270	16,270	16,179
Stock Issued During Period, Value, Acquisitions											6,478
Stock Issued During Period, Value, Conversion of Convertible Securities, Net of Adjustments									(9)
Stock Issued During Period, Value, New Issues									2,909
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance				12,913				12,596	12,913	12,596	13,111
Net Income									874	1,233	311
Cash dividends, common									(888)	(832)	(775)
Dividends, Preferred Stock									(107)	(152)	(152)
Share-based compensation and other plans, net									0	(1)	(2)
Stockholders' Equity, Other										68	103
Ending balance	12,791				12,913				12,791	12,913	12,596
Deferred Compensation
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance				23				22	23	22	19
Share-based compensation and other plans, net									0	1	3
Ending balance	23				23				23	23	22
Treasury Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance				$ (6,190)				$ (6,243)	$ (6,190)	$ (6,243)	$ (8,427)
Beginning balance (shares)				(76,260,000)				(78,463,000)	(76,260,000)	(78,463,000)	(118,745,000)
Share-based compensation and other plans, net									$ 52	$ 53	$ 62
Share-based compensation plans, net (shares)									1,597,000	2,155,000	2,982,000
Common stock held in trusts, net (shares)									41,000	48,000	(6,000)
Stock Issued During Period, Shares, Acquisitions											37,306,000
Ending balance	$ (6,138)				$ (6,190)				$ (6,138)	$ (6,190)	$ (6,243)
Ending balance (shares)	(74,623,000)				(76,260,000)				(74,623,000)	(76,260,000)	(78,463,000)
Stock Issued During Period, Value, Acquisitions											$ 2,121