Earnings Per Common Share - Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions		3 Months Ended
		Mar. 31, 2022	Mar. 31, 2021
EARNINGS
Income (loss) from continuing operations		$ 447	$ 618
Less: Net income (loss) attributable to noncontrolling interests		0	0
Income (loss) from continuing operations attributable to Key		447	618
Less: Dividends on Preferred Stock		27	27
Income (loss) from continuing operations attributable to Key common shareholders		420	591
Income (loss) from discontinued operations, net of taxes		1	4
Net income (loss) attributable to Key common shareholders		$ 421	$ 595
WEIGHTED-AVERAGE COMMON SHARES
Weighted-average common shares outstanding (000) (in shares)		922,941	964,878
Effect of common share options and other stock awards (in shares)		10,692	9,419
Weighted-average common shares and potential common shares outstanding (in shares)	[1]	933,634	974,297
EARNINGS PER COMMON SHARE
Income (loss) from continuing operations attributable to Key common shareholders (in usd per share)		$ 0.45	$ 0.61
Income (loss) from discontinued operations, net of taxes (in usd per share)		0	0
Net income (loss) attributable to Key common shareholders (in usd per share)	[2]	0.46	0.62
Income (loss) from continuing operations attributable to Key common shareholders - assuming dilution (in usd per share)		0.45	0.61
Income (loss) from discontinued operations, net of taxes - assuming dilution (in usd per share)		0	0
Net income (loss) attributable to Key common shareholders (in usd per share)	[2]	$ 0.45	$ 0.61

[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.