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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
EARNINGS
Income (loss) from continuing operations		$ 185	$ 423	$ 330	$ 829
Less: Net income (loss) attributable to noncontrolling interests		0	0	0	0
Income (loss) from continuing operations attributable to Key		185	423	330	829
Less: Dividends on Preferred Stock		26	20	53	40
Income (loss) from continuing operations attributable to Key common shareholders		159	403	277	789
Income (loss) from discontinued operations, net of taxes		2	2	3	3
Net income (loss) attributable to Key common shareholders		$ 161	$ 405	$ 280	$ 792
WEIGHTED-AVERAGE COMMON SHARES
Weighted-average common shares outstanding (000) (in shares)		967,147	999,163	967,380	1,003,047
Effect of common share options and other stock awards (in shares)		4,994	8,801	6,892	9,318
Weighted-average common shares and potential common shares outstanding (in shares)	[1]	972,141	1,007,964	974,272	1,012,365
EARNINGS PER COMMON SHARE
Income (loss) from continuing operations attributable to Key common shareholders (in usd per share)		$ 0.16	$ 0.40	$ 0.29	$ 0.79
Income (loss) from discontinued operations, net of taxes (in usd per share)		0	0	0	0
Net income (loss) attributable to Key common shareholders (in usd per share)	[2]	0.17	0.40	0.29	0.79
Income (loss) from continuing operations attributable to Key common shareholders - assuming dilution (in usd per share)		0.16	0.40	0.28	0.78
Income (loss) from discontinued operations, net of taxes - assuming dilution (in usd per share)		0	0	0	0
Net income (loss) attributable to Key common shareholders (in usd per share)	[2]	$ 0.17	$ 0.40	$ 0.29	$ 0.78

[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.