Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total	Preferred Shares [Member]	Common Shares [Member]	Additional Paid-in Capital [Member]	AOCI Attributable to Parent [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	3,257					3,257
Other comprehensive income (loss)	(55)				(55)	0
Repurchase of common shares	0					0
Other changes, primarily employee plans	75		1	96		(22)
Cash dividends declared preferred							$ (20)	$ (20)	$ (21)	$ (21)
Cash dividends declared common	(605)					(605)
Ending Balance at Jun. 30, 2018	20,892	0	173	12,306	(2,483)	10,896
Beginning Balance at Mar. 31, 2018	19,613	0	172	12,225	(2,381)	9,597
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,623					1,623
Other comprehensive income (loss)	(102)				(102)	0
Repurchase of common shares	0					0
Other changes, primarily employee plans	82		1	81		0
Cash dividends declared preferred							(20)	(20)
Cash dividends declared common	(304)					(304)
Ending Balance at Jun. 30, 2018	20,892	0	173	12,306	(2,483)	10,896
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	3,311					3,311
Other comprehensive income (loss)	(9)				(9)	0
Repurchase of common shares	(1,875)		(4)	(347)		(1,524)
Other changes, primarily employee plans	69		1	109		(41)
Cash dividends declared preferred							(19)	(19)	$ (21)	$ (21)
Cash dividends declared common	(654)					(654)
Ending Balance at Jun. 30, 2019	23,092	0	167	11,980	(2,606)	13,551
Beginning Balance at Mar. 31, 2019	22,218	0	168	11,963	(2,599)	12,686
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,761					1,761
Other comprehensive income (loss)	(7)				(7)	0
Repurchase of common shares	(630)		(1)	(80)		(549)
Other changes, primarily employee plans	96			97		(1)
Cash dividends declared preferred							$ (19)	$ (19)
Cash dividends declared common	(327)					(327)
Ending Balance at Jun. 30, 2019	$ 23,092	$ 0	$ 167	$ 11,980	$ (2,606)	$ 13,551