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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
EARNINGS
Income (loss) from continuing operations		$ 424	$ 413	$ 754	$ 1,242
Less: Net income (loss) attributable to noncontrolling interests		0	0	0	0
Income (loss) from continuing operations attributable to Key		424	413	754	1,242
Less: Dividends on Preferred Stock		27	30	80	70
Income (loss) from continuing operations attributable to Key common shareholders		397	383	674	1,172
Income (loss) from discontinued operations, net of taxes		4	3	7	6
Net income (loss) attributable to Key common shareholders		$ 401	$ 386	$ 681	$ 1,178
WEIGHTED-AVERAGE COMMON SHARES
Weighted-average common shares outstanding (000) (in shares)		967,804	988,319	967,632	998,268
Effect of common share options and other stock awards (in shares)		6,184	10,009	6,648	9,632
Weighted-average common shares and potential common shares outstanding (in shares)	[1]	973,988	998,328	974,280	1,007,900
EARNINGS PER COMMON SHARE
Income (loss) from continuing operations attributable to Key common shareholders (in usd per share)		$ 0.41	$ 0.39	$ 0.70	$ 1.17
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]	0.41	0.39	0.70	1.18
Income (loss) from continuing operations attributable to Key common shareholders - assuming dilution (in usd per share)		0.41	0.38	0.69	1.16
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]	$ 0.41	$ 0.39	$ 0.70	$ 1.17

[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.