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, 2020	Mar. 31, 2019
EARNINGS
Income (loss) from continuing operations		$ 145	$ 406
Less: Net income (loss) attributable to noncontrolling interests		0	0
Income (loss) from continuing operations attributable to Key		145	406
Less: Dividends on Preferred Stock		27	20
Income (loss) from continuing operations attributable to Key common shareholders		118	386
Income (loss) from discontinued operations, net of taxes		1	1
Net income (loss) attributable to Key common shareholders		$ 119	$ 387
WEIGHTED-AVERAGE COMMON SHARES
Weighted-average common shares outstanding (000) (in shares)		967,446	1,006,717
Effect of common share options and other stock awards (in shares)		8,664	9,787
Weighted-average common shares and potential common shares outstanding (in shares)	[1]	976,110	1,016,504
EARNINGS PER COMMON SHARE
Income (loss) from continuing operations attributable to Key common shareholders (in usd per share)		$ 0.12	$ 0.38
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]	0.12	0.38
Income (loss) from continuing operations attributable to Key common shareholders - assuming dilution (in usd per share)		0.12	0.38
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]	$ 0.12	$ 0.38

[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.