Earnings Per Common Share - Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
EARNINGS
Income (loss) from continuing operations		$ 724	$ 185	$ 1,342	$ 330
Less: Net income (loss) attributable to noncontrolling interests		0	0	0	0
Income (loss) from continuing operations attributable to Key		724	185	1,342	330
Less: Dividends on Preferred Stock		26	26	53	53
Income (loss) from continuing operations attributable to Key common shareholders		698	159	1,289	277
Income (loss) from discontinued operations, net of taxes		5	2	9	3
Net income (loss) attributable to Key common shareholders		$ 703	$ 161	$ 1,298	$ 280
WEIGHTED-AVERAGE COMMON SHARES
Weighted-average common shares outstanding (000) (in shares)		957,423	967,147	961,292	967,380
Effect of common share options and other stock awards (in shares)		9,740	4,994	9,514	6,892
Weighted-average common shares and potential common shares outstanding (in shares)	[1]	967,163	972,141	970,806	974,272
EARNINGS PER COMMON SHARE
Income (loss) from continuing operations attributable to Key common shareholders (in usd per share)		$ 0.73	$ 0.16	$ 1.34	$ 0.29
Income (loss) from discontinued operations, net of taxes (in usd per share)		0	0	0.01	0
Net income (loss) attributable to Key common shareholders (in usd per share)	[2]	0.73	0.17	1.35	0.29
Income (loss) from continuing operations attributable to Key common shareholders - assuming dilution (in usd per share)		0.72	0.16	1.33	0.28
Income (loss) from discontinued operations, net of taxes - assuming dilution (in usd per share)		0	0	0.01	0
Net income (loss) attributable to Key common shareholders (in usd per share)	[2]	$ 0.73	$ 0.17	$ 1.34	$ 0.29

[1] Assumes conversion of Common Share options and other stock awards and/or convertible preferred stock, as applicable.

[2] EPS may not foot due to rounding.