Shareholders' Equity - Changes in Certain Components of Shareholders' Equity (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance				$ 12,948					$ 12,948
Beginning balance (shares)				(118,744,758)					(118,744,758)
Net Income	$ (135)	$ 594	$ (12)	$ (136)	$ 327	$ (132)	$ 344	$ 562	$ 311	$ 1,100	$ 976
Ending balance	$ 20,994				$ 12,948				$ 20,994	$ 12,948
Ending balance (shares)	(78,462,971)				(118,744,758)				(78,462,971)	(118,744,758)
Common Stock Issued at Par Value
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance				347				333	$ 347	$ 333	333
Stock Issued During Period, Value, New Issues										14
Ending balance	$ 347				$ 347				347	347	333
Capital in Excess of Par Value
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance				9,619				4,693	9,619	4,693	4,475
Stock Issued During Period, Value, Acquisitions									6,478
Stock Issued During Period, Value, New Issues										4,810
Share-based compensation and other plans, net									(246)	(65)	27
Share-based compensation									328	180	191
Ending balance	16,179				9,619				16,179	9,619	4,693
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance				13,111				12,727	13,111	12,727	12,314
Net Income									311	1,100	976
Cash dividends, common									(775)	(645)	(562)
Dividends, Preferred Stock									(152)	(70)
Share-based compensation and other plans, net									(2)	(1)	(1)
Effect of change in accounting principle (see Note 2 and further discussion below)	103								103
Ending balance	12,596				13,111				12,596	13,111	12,727
Deferred Compensation
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance				19				22	19	22	20
Share-based compensation and other plans, net									3	(3)	2
Ending balance	22				19				22	19	22
Treasury Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance				$ (8,427)				$ (8,212)	$ (8,427)	$ (8,212)	$ (8,239)
Beginning balance (shares)				(118,745,000)				(119,371,000)	(118,745,000)	(119,371,000)	(121,967,000)
Stock Issued During Period, Value, Acquisitions									$ 2,121
Stock Issued During Period, Shares, Acquisitions									37,306,000
Share-based compensation and other plans, net									$ 62	$ 6	$ 26
Share-based compensation plans, net (shares)									2,982,000	1,908,000	2,607,000
Common stock held in trusts, net (shares)									(6,000)	7,000	(11,000)
Repurchase of common stock (shares)										(1,289,000)
Repurchase of common stock										$ (220)
Ending balance	$ (6,243)				$ (8,427)				$ (6,243)	$ (8,427)	$ (8,212)
Ending balance (shares)	(78,463,000)				(118,745,000)				(78,463,000)	(118,745,000)	(119,371,000)