Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total	Preferred Shares [Member]	Common Shares [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Retained Earnings [Member]	Series B Preferred Stock [Member]	Series B Preferred Stock [Member] Retained Earnings [Member]	Series C Preferred Stock [Member]	Series C Preferred Stock [Member] Retained Earnings [Member]
Beginning Balance at Dec. 31, 2017	$ 18,261	$ 0	$ 172	$ 12,210	$ (2,428)	$ 8,307
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	4,911					4,911
Other comprehensive loss	(76)				(76)	0
Repurchase of common shares	(780)		(2)	(105)		(673)
Other changes, primarily employee plans	145		1	167		(23)
Cash dividends declared preferred							$ (20)	$ (20)	$ (41)	$ (41)
Cash dividends declared common	(940)					(940)
Ending Balance at Sep. 30, 2018	21,460	0	171	12,272	(2,504)	11,521
Beginning Balance at Jun. 30, 2018	20,892	0	173	12,306	(2,483)	10,896
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,654					1,654
Other comprehensive loss	(21)				(21)	0
Repurchase of common shares	(780)		(2)	(105)		(673)
Other changes, primarily employee plans	70		0	71		(1)
Cash dividends declared preferred									(20)	(20)
Cash dividends declared common	(335)					(335)
Ending Balance at Sep. 30, 2018	21,460	0	171	12,272	(2,504)	11,521
Beginning Balance at Dec. 31, 2018	22,290	0	170	12,218	(2,597)	12,499
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	5,066					5,066
Other comprehensive loss	(64)				(64)	0
Repurchase of common shares	(3,325)		(6)	(517)		(2,802)
Other changes, primarily employee plans	127		1	168		(42)
Cash dividends declared preferred							$ (19)	$ (19)	(42)	(42)
Cash dividends declared common	(1,008)					(1,008)
Ending Balance at Sep. 30, 2019	23,025	0	165	11,869	(2,661)	13,652
Beginning Balance at Jun. 30, 2019	23,092	0	167	11,980	(2,606)	13,551
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,755					1,755
Other comprehensive loss	(55)				(55)	0
Repurchase of common shares	(1,450)		(2)	(170)		(1,278)
Other changes, primarily employee plans	58		0	59		(1)
Cash dividends declared preferred									$ (21)	$ (21)
Cash dividends declared common	(354)					(354)
Ending Balance at Sep. 30, 2019	$ 23,025	$ 0	$ 165	$ 11,869	$ (2,661)	$ 13,652