CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (UNAUDITED) - USD ($) shares in Millions, $ in Millions		Total	Cumulative Effect, Period of Adoption, Adjustment	Series B convertible participating preferred stock	Series C convertible participating preferred stock	Class A common stock	Preferred Stock	Preferred Stock Conversion Of Series A Preferred Stock Upon Sale Into Public Market	Preferred Stock Series A convertible participating preferred stock	[1]	Preferred Stock Series A convertible participating preferred stock Conversion Of Series A Preferred Stock Upon Sale Into Public Market	[1]	Preferred Stock Series B convertible participating preferred stock	Preferred Stock Series C convertible participating preferred stock	Common Stock Class A common stock	Common Stock Class A common stock Conversion Of Series A Preferred Stock Upon Sale Into Public Market	Common Stock Class A common stock Conversion Of Class C Common Stock Upon Sale Into Public Market	[1]	Common Stock Class B common stock	Common Stock Class C common stock		Common Stock Class C common stock Conversion Of Class C Common Stock Upon Sale Into Public Market	[1]	Right to Recover for Covered Losses	Additional Paid-In Capital	Additional Paid-In Capital Conversion Of Series A Preferred Stock Upon Sale Into Public Market	Accumulated Income	Accumulated Income Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Income (Loss), Net	Accumulated Other Comprehensive Income (Loss), Net Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of new accounting standards		$ 34,684					$ 5,462																	$ (171)	$ 16,541		$ 13,502	$ 25	$ (650)	$ (25)
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	25	(650)	(25)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		3,272																									3,272
Other comprehensive income (loss), net of tax		335																											335
Comprehensive income		3,607
Adoption of new accounting standards		35,270					5,462																	(175)	16,424		13,899	25	(340)	$ (25)
VE territory covered losses incurred		(4)																						(4)
Conversion of stock (in shares)															1					0	[1]
Vesting of restricted stock and performance-based shares (in shares)															3
Share-based compensation, net of forfeitures		116																							116
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		(147)																							(147)
Cash proceeds from issuance of common stock under employee equity plans (in shares)															1
Cash proceeds from issuance of class A common stock under employee equity plans		55																							55
Cash dividends declared and paid, at a quarterly amount per Class A share		(671)																									(671)
Repurchase of class A common stock (in shares)						(13)									(13)
Repurchase of class A common stock		(2,370)				$ (2,370)																			(141)		(2,229)
Ending balance (in shares) at Dec. 31, 2019													2	3	1,709				245	11
Ending balance at Dec. 31, 2019		35,270					5,462																	(175)	16,424		13,899		(340)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of new accounting standards		35,270					5,462																	(175)	16,424		13,899		(340)
Adoption of new accounting standards		36,210	$ 3				5,086						$ 1,106	$ 1,543										(39)	16,721		14,088	3	354
Beginning balance (in shares) at Sep. 30, 2020									0				2	3	1,683				245	11
Beginning balance at Sep. 30, 2020		36,210	3				5,086						$ 1,106	$ 1,543										(39)	16,721		14,088	3	354
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		3,126																									3,126
Other comprehensive income (loss), net of tax		800																											800
Comprehensive income		3,926
Adoption of new accounting standards		36,210	$ 3				3,683						1,097	1,537										(39)	18,063		14,813	$ 3	1,154
VE territory covered losses incurred		(10)											0	0										(10)
Recovery through conversion rate adjustment				$ 9	$ 6		(15)						$ 9	$ 6										15
Conversion of stock (in shares)											0					20	0					0
Conversion of stock								$ (1,388)																		$ 1,388
Vesting of restricted stock and performance-based shares (in shares)															3
Share-based compensation, net of forfeitures		122																							122
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		(134)																							(134)
Cash proceeds from issuance of common stock under employee equity plans (in shares)	[1]														0
Cash proceeds from issuance of class A common stock under employee equity plans		61																							61
Cash dividends declared and paid, at a quarterly amount per Class A share		(703)																									(703)
Repurchase of class A common stock (in shares)						(9)									(9)
Repurchase of class A common stock		(1,796)				$ (1,796)																			(95)		(1,701)
Ending balance (in shares) at Dec. 31, 2020									0				2	3	1,696				245	11
Ending balance at Dec. 31, 2020		37,679					3,683						$ 1,097	$ 1,537										(34)	18,063		14,813		1,154
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of new accounting standards		$ 37,679					$ 3,683						$ 1,097	$ 1,537										$ (34)	$ 18,063		$ 14,813		$ 1,154

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