CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (UNAUDITED) - USD ($) shares in Millions, $ in Millions		Total	Class A common stock	Preferred Stock	Preferred Stock UK&I preferred stock	Preferred Stock Europe preferred stock	Common Stock Class A common stock	Common Stock Class B common stock	Common Stock Class C common stock		Right to Recover for Covered Losses	Additional Paid-In Capital	Accumulated Income	Accumulated Other Comprehensive Income (Loss), Net
Beginning balance (in shares) at Sep. 30, 2018					2	3	1,768	245	12
Beginning balance at Sep. 30, 2018		$ 34,006		$ 5,470							$ (7)	$ 16,678	$ 11,318	$ 547
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		5,954											5,954
Other comprehensive income (loss), net of tax		(640)												(640)
Comprehensive income		5,314
Adoption of new accounting standards (Note 1)		392											385	7
VE territory covered losses incurred (Note 5)		(162)									(162)
Recovery through conversion rate adjustment (Note 5 and Note 10)				(6)							6
Conversion of class C common stock upon sales into public market (in shares)							1		0	[1]
Vesting of restricted stock and performance-based shares (in shares)							3
Share-based compensation, net of forfeitures (Note 12)		211										211
Restricted stock and performance-based shares settled in cash for taxes (in shares)							(1)
Restricted stock and performance-based shares settled in cash for taxes		(103)										(103)
Cash proceeds from issuance of common stock under employee equity plans (in shares)							1
Cash proceeds from issuance of common stock under employee equity plans		89										89
Cash dividends declared and paid, at a quarterly amount per Class A share		(1,141)											(1,141)
Repurchase of class A common stock (Note 10) (in shares)			(31)				(31)
Repurchase of class A common stock (Note 10)		(4,331)	$ (4,331)									(328)	(4,003)
Ending balance (in shares) at Mar. 31, 2019					2	3	1,741	245	12
Ending balance at Mar. 31, 2019		34,275		5,464							(163)	16,547	12,513	(86)
Beginning balance (in shares) at Dec. 31, 2018					2	3	1,754	245	12
Beginning balance at Dec. 31, 2018		34,095		5,464							(92)	16,540	11,908	275
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		2,977											2,977
Other comprehensive income (loss), net of tax		(361)												(361)
Comprehensive income		2,616
Adoption of new accounting standards (Note 1)		(8)											(8)
VE territory covered losses incurred (Note 5)		(71)									(71)
Conversion of class C common stock upon sales into public market (in shares)							1		0	[1]
Vesting of restricted stock and performance-based shares (in shares)	[1]						0
Share-based compensation, net of forfeitures (Note 12)		111										111
Restricted stock and performance-based shares settled in cash for taxes (in shares)	[1]						0
Restricted stock and performance-based shares settled in cash for taxes		(2)										(2)
Cash proceeds from issuance of common stock under employee equity plans (in shares)	[1]						0
Cash proceeds from issuance of common stock under employee equity plans		41										41
Cash dividends declared and paid, at a quarterly amount per Class A share		(569)											(569)
Repurchase of class A common stock (Note 10) (in shares)			(14)				(14)
Repurchase of class A common stock (Note 10)		(1,938)	$ (1,938)									(143)	(1,795)
Ending balance (in shares) at Mar. 31, 2019					2	3	1,741	245	12
Ending balance at Mar. 31, 2019		34,275		5,464							(163)	16,547	12,513	(86)
Beginning balance (in shares) at Sep. 30, 2019					2	3	1,718	245	11
Beginning balance at Sep. 30, 2019		34,684		5,462							(171)	16,541	13,502	(650)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		6,356											6,356
Other comprehensive income (loss), net of tax		231												231
Comprehensive income		6,587
Adoption of new accounting standards (Note 1)		0											25	(25)
VE territory covered losses incurred (Note 5)		(13)									(13)
Conversion of class C common stock upon sales into public market (in shares)							3		0
Vesting of restricted stock and performance-based shares (in shares)							3
Share-based compensation, net of forfeitures (Note 12)		215										215
Restricted stock and performance-based shares settled in cash for taxes (in shares)							1
Restricted stock and performance-based shares settled in cash for taxes		(155)										(155)
Cash proceeds from issuance of common stock under employee equity plans (in shares)							1
Cash proceeds from issuance of common stock under employee equity plans		109										109
Cash dividends declared and paid, at a quarterly amount per Class A share		(1,339)											(1,339)
Repurchase of class A common stock (Note 10) (in shares)			(31)				(31)
Repurchase of class A common stock (Note 10)		(5,503)	$ (5,503)									(325)	(5,178)
Ending balance (in shares) at Mar. 31, 2020					2	3	1,693	245	11
Ending balance at Mar. 31, 2020		34,585		5,462							(184)	16,385	13,366	(444)
Beginning balance (in shares) at Dec. 31, 2019					2	3	1,709	245	11
Beginning balance at Dec. 31, 2019		35,270		5,462							(175)	16,424	13,899	(340)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		3,084											3,084
Other comprehensive income (loss), net of tax		(104)												(104)
Comprehensive income		2,980
VE territory covered losses incurred (Note 5)		(9)									(9)
Conversion of class C common stock upon sales into public market (in shares)							2		0	[1]
Vesting of restricted stock and performance-based shares (in shares)							0
Share-based compensation, net of forfeitures (Note 12)		99										99
Restricted stock and performance-based shares settled in cash for taxes (in shares)							0
Restricted stock and performance-based shares settled in cash for taxes		(8)										(8)
Cash proceeds from issuance of common stock under employee equity plans (in shares)							0
Cash proceeds from issuance of common stock under employee equity plans		54										54
Cash dividends declared and paid, at a quarterly amount per Class A share		(668)											(668)
Repurchase of class A common stock (Note 10) (in shares)			(18)				(18)
Repurchase of class A common stock (Note 10)		(3,133)	$ (3,133)									(184)	(2,949)
Ending balance (in shares) at Mar. 31, 2020					2	3	1,693	245	11
Ending balance at Mar. 31, 2020		$ 34,585		$ 5,462							$ (184)	$ 16,385	$ 13,366	$ (444)

[1] Increase or decrease is less than one million shares.