CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) shares in Millions, $ in Millions		Total	U.K.& I preferred stock	Europe preferred stock	Class A common stock	[3]	Preferred Stock	Preferred Stock U.K.& 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
Beginning balance (in shares) at Sep. 30, 2017								2	3	1,818	245	13
Beginning balance at Sep. 30, 2017		$ 32,760					$ 5,526							$ (52)	$ 16,900	$ 9,508	$ 878
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		5,127
Other comprehensive income (loss), net of tax		850															850
Comprehensive income		5,977
VE territory covered losses incurred (Note 4)		(4)												(4)
Recovery through conversion rate adjustment (Note 4 and Note 9)		0					(50)							50
Conversion of class C common stock upon sales into public market (in shares)										2		(1)
Vesting of restricted stock and performance-based shares (in shares)										2
Stock Issued During Period, Shares, Restricted Stock Award, Forfeited	[1]									0
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition		153													153
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		(88)													(88)
Cash proceeds from issuance of common stock under employee equity plans (in shares)										2
Cash proceeds from issuance of common stock under employee equity plans		103													103
Cash dividends declared and paid, at a quarterly amount per Class A share		(948)														(948)
Repurchase of class A common stock (Note 9) (in shares)					(33)	[2]				(33)
Repurchase of class A common stock (Note 9)		(3,850)			$ (3,850)										(355)	(3,495)
Ending balance (in shares) at Mar. 31, 2018								2	3	1,790	245	12
Ending balance at Mar. 31, 2018		34,103					5,476							(6)	16,713	10,192	1,728
Beginning balance (in shares) at Sep. 30, 2017								2	3	1,818	245	13
Beginning balance at Sep. 30, 2017		32,760					5,526							(52)	16,900	9,508	878
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Recovery through conversion rate adjustment (Note 4 and Note 9)			$ 35	$ 21
Ending balance (in shares) at Sep. 30, 2018								2	3	1,768	245	12
Ending balance at Sep. 30, 2018		34,006					5,470							(7)	16,678	11,318	547
Beginning balance (in shares) at Dec. 31, 2017								2	3	1,805	245	12
Beginning balance at Dec. 31, 2017		33,401					5,476							(5)	16,761	9,966	1,203
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		2,605
Other comprehensive income (loss), net of tax		525															525
Comprehensive income		3,130
VE territory covered losses incurred (Note 4)		(1)												(1)
Conversion of class C common stock upon sales into public market (in shares)										1		0	[4]
Vesting of restricted stock and performance-based shares (in shares)	[4]									0
Stock Issued During Period, Shares, Restricted Stock Award, Forfeited	[4]									0
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition		85													85
Restricted stock and performance-based shares settled in cash for taxes (in shares)	[4]									0
Restricted stock and performance-based shares settled in cash for taxes		0													0
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		50													50
Cash dividends declared and paid, at a quarterly amount per Class A share		(490)														(490)
Repurchase of class A common stock (Note 9) (in shares)					(17)	[2]				(17)
Repurchase of class A common stock (Note 9)		(2,072)			$ (2,072)										(183)	(1,889)
Ending balance (in shares) at Mar. 31, 2018								2	3	1,790	245	12
Ending balance at Mar. 31, 2018		34,103					5,476							(6)	16,713	10,192	1,728
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
Other comprehensive income (loss), net of tax		(640)															(640)
Comprehensive income		5,314
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		392														385	7
VE territory covered losses incurred (Note 4)		(162)	0	0										(162)
Recovery through conversion rate adjustment (Note 4 and Note 9)		0	$ 5	$ 1			(6)							6
Conversion of class C common stock upon sales into public market (in shares)										1		0	[5]
Vesting of restricted stock and performance-based shares (in shares)										3
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition		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 9) (in shares)					(31)	[2]				(31)
Repurchase of class A common stock (Note 9)		(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
Other comprehensive income (loss), net of tax		(361)															(361)
Comprehensive income		2,616
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification		(8)														(8)	0
VE territory covered losses incurred (Note 4)		(71)												(71)
Conversion of class C common stock upon sales into public market (in shares)										1		0	[6]
Vesting of restricted stock and performance-based shares (in shares)	[6]									0
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition		111													111
Restricted stock and performance-based shares settled in cash for taxes (in shares)	[6]									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)	[6]									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 9) (in shares)					(14)	[2]				(14)
Repurchase of class A common stock (Note 9)		(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)

[1] Decrease is less than one million shares.

[2] All shares repurchased in the open market have been retired and constitute authorized but unissued shares.

[3] Figures in the table may not recalculate exactly due to rounding. Shares repurchased in the open market reflect repurchases settled during the three and six months ended March 31, 2019 and 2018. These amounts include repurchases traded but not yet settled on or before September 30, 2018 and 2017 for six months, respectively, and December 31, 2018 and 2017 for three months, respectively. Also, these amounts exclude repurchases traded but not yet settled on or before March 31, 2019 and 2018, respectively.

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

[5] Decrease is less than one million shares.

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