Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions		3 Months Ended
		Mar. 31, 2024	Mar. 31, 2023
EARNINGS
Income (loss) from continuing operations		$ 219	$ 311
Less: Net income (loss) attributable to noncontrolling interests		0	0
Income (loss) from continuing operations attributable to Key		219	311
Less: Dividends on Preferred Stock		36	36
Income (loss) from continuing operations attributable to Key common shareholders		183	275
Income (loss) from discontinued operations, net of taxes		0	1
Net income (loss) attributable to Key common shareholders		$ 183	$ 276
WEIGHTED-AVERAGE COMMON SHARES
Weighted-average Common Shares outstanding (in shares)		929,692	926,490
Effect of Common Share options and other stock awards (in shares)		7,319	7,314
Weighted-average Common Shares and potential Common Shares outstanding (in shares)	[1]	937,011	933,804
EARNINGS PER COMMON SHARE
Income (loss) from continuing operations attributable to Key common shareholders (in usd per share)		$ 0.20	$ 0.30
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]	0.20	0.30
Income (loss) from continuing operations attributable to Key common shareholders - assuming dilution (in usd per share)		0.20	0.30
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]	$ 0.20	$ 0.30

[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.