Earnings Per Common Share - Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
EARNINGS
Income (loss) from continuing operations		$ 643	$ 424	$ 1,985	$ 754
Less: Net income (loss) attributable to noncontrolling interests		0	0	0	0
Income (loss) from continuing operations attributable to Key		643	424	1,985	754
Less: Dividends on Preferred Stock		27	27	80	80
Income (loss) from continuing operations attributable to Key common shareholders		616	397	1,905	674
Income (loss) from discontinued operations, net of taxes		2	4	11	7
Net income (loss) attributable to Key common shareholders		$ 618	$ 401	$ 1,916	$ 681
WEIGHTED-AVERAGE COMMON SHARES
Weighted-average common shares outstanding (000) (in shares)		942,446	967,804	955,069	967,632
Effect of common share options and other stock awards (in shares)		10,077	6,184	9,712	6,648
Weighted-average common shares and potential common shares outstanding (in shares)	[1]	952,523	973,988	964,781	974,280
EARNINGS PER COMMON SHARE
Income (loss) from continuing operations attributable to Key common shareholders (in usd per share)		$ 0.65	$ 0.41	$ 1.99	$ 0.70
Income (loss) from discontinued operations, net of taxes (in usd per share)		0	0	0.01	0.01
Net income (loss) attributable to Key common shareholders (in usd per share)	[2]	0.66	0.41	2.00	0.70
Income (loss) from continuing operations attributable to Key common shareholders - assuming dilution (in usd per share)		0.65	0.41	1.98	0.69
Income (loss) from discontinued operations, net of taxes - assuming dilution (in usd per share)		0	0	0.01	0.01
Net income (loss) attributable to Key common shareholders (in usd per share)	[2]	$ 0.65	$ 0.41	$ 1.99	$ 0.70

[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.