Shareholders' Equity - Changes in Certain Components of Shareholders' Equity (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance				$ 20,994					$ 20,994
Beginning balance (shares)				(78,462,971)					(78,462,971)
Net Income	$ 163	$ 451	$ 20	$ 599	$ (135)	$ 594	$ (12)	$ (136)	$ 1,233	$ 311	$ 1,100
Ending balance	$ 21,081				$ 20,994				$ 21,081	$ 20,994
Ending balance (shares)	(76,259,835)				(78,462,971)				(76,259,835)	(78,462,971)
Common Stock Issued at Par Value
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance				347				347	$ 347	$ 347	333
Stock Issued During Period, Value, New Issues											14
Ending balance	$ 347				$ 347				347	347	347
Capital in Excess of Par Value
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance				16,179				9,619	16,179	9,619	4,693
Stock Issued During Period, Value, New Issues											4,810
Share-based compensation and other plans, net									(170)	(246)	(65)
Share-based compensation									261	328	180
Stock Issued During Period, Value, Acquisitions										6,478
Ending balance	16,270				16,179				16,270	16,179	9,619
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance				12,596				13,111	12,596	13,111	12,727
Net Income									1,233	311	1,100
Cash dividends, common									(832)	(775)	(645)
Dividends, Preferred Stock									(152)	(152)	(70)
Share-based compensation and other plans, net									(1)	(2)	(1)
Effect of change in accounting principle (see Note 2)	68				103				68	103
Ending balance	12,913				12,596				12,913	12,596	13,111
Deferred Compensation
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance				22				19	22	19	22
Share-based compensation and other plans, net									1	3	(3)
Ending balance	23				22				23	22	19
Treasury Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance				$ (6,243)				$ (8,427)	$ (6,243)	$ (8,427)	$ (8,212)
Beginning balance (shares)				(78,463,000)				(118,745,000)	(78,463,000)	(118,745,000)	(119,371,000)
Share-based compensation and other plans, net									$ 53	$ 62	$ 6
Share-based compensation plans, net (shares)									2,155,000	2,982,000	1,908,000
Common stock held in trusts, net (shares)									48,000	(6,000)	7,000
Repurchase of common stock (shares)											1,289,000
Repurchase of common stock											$ 220
Stock Issued During Period, Value, Acquisitions										$ 2,121
Stock Issued During Period, Shares, Acquisitions										37,306,000
Ending balance	$ (6,190)				$ (6,243)				$ (6,190)	$ (6,243)	$ (8,427)
Ending balance (shares)	(76,260,000)				(78,463,000)				(76,260,000)	(78,463,000)	(118,745,000)