Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total	Cumulative Effect, Period of Adoption, Adjustment [Member]		Series B [Member]	Series C [Member]	Preferred Shares [Member]	Common Shares [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Retained Earnings [Member]	Retained Earnings [Member] Cumulative Effect, Period of Adoption, Adjustment [Member]		Retained Earnings [Member] Series B [Member]	Retained Earnings [Member] Series C [Member]
Beginning Balance at Dec. 31, 2017	$ 18,261					$ 0	$ 172	$ 12,210	$ (2,428)	$ 8,307
Net income (loss)	6,921									6,921
Other comprehensive (loss) income	(169)								(169)	0
Repurchase of common shares	(1,570)						(3)	(216)		(1,351)
Other changes, primarily employee plans	200						1	224		(25)
Cash dividends declared preferred				$ (39)	$ (41)								$ (39)	$ (41)
Cash dividends declared common	(1,273)									(1,273)
Ending Balance at Dec. 31, 2018	22,290					0	170	12,218	(2,597)	12,499
Net income (loss)	6,759									6,759
Other comprehensive (loss) income	(140)								(140)	0
Repurchase of common shares	(4,585)						(8)	(671)		(3,906)
Other changes, primarily employee plans	186						1	227		(42)
Cash dividends declared preferred				(39)	(42)								(39)	(42)
Cash dividends declared common	(1,358)									(1,358)
Ending Balance at Dec. 31, 2019	$ 23,071	$ (882)	[1]			0	163	11,774	(2,737)	13,871	$ (882)	[1]
Ending Balance at Jan. 01, 2020											(882)
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Impact of new accounting guidance, tax		288
Beginning Balance at Dec. 31, 2019	$ 23,071	(882)	[1]			0	163	11,774	(2,737)	13,871	$ (882)	[1]
Net income (loss)	3,135									3,135
Other comprehensive (loss) income	(158)								(158)	0
Repurchase of common shares	(875)						(2)	(105)		(768)
Other changes, primarily employee plans	164						0	212		(48)
Cash dividends declared preferred				$ (34)	$ (45)								$ (34)	$ (45)
Cash dividends declared common	(1,392)									(1,392)
Ending Balance at Dec. 31, 2020	$ 22,984					$ 0	$ 161	$ 11,881	$ (2,895)	$ 13,837
Impact of new accounting guidance, net of tax		$ 1,170

[1] Represents $1,170 million, net of tax of $288 million, related to the impact as of January 1, 2020 of adopting the new accounting guidance for the recognition of credit losses on certain financial instruments.