Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions		12 Months Ended
		Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
EARNINGS
Income (loss) from continuing operations		$ (163)	$ 964	$ 1,911
Less: Dividends on preferred stock		143	143	118
Income (loss) from continuing operations attributable to Key common shareholders		(306)	821	1,793
Income (loss) from discontinued operations, net of taxes		2	3	6
Net income (loss) attributable to Key common shareholders		$ (304)	$ 824	$ 1,799
WEIGHTED-AVERAGE COMMON SHARES
Weighted-average Common Shares outstanding (in shares)		949,561	927,217	924,363
Effect of common share options and other stock awards (in shares)	[1]	0	5,542	8,696
Weighted-average Common Shares and potential Common Shares outstanding (in shares)	[2]	949,561	932,759	933,059
EARNINGS PER COMMON SHARE
Income (loss) from continuing operations attributable to Key common shareholders (in dollars per share)		$ (0.32)	$ 0.88	$ 1.94
Income (loss) from discontinued operations, net of taxes (in dollars per share)		0	0	0.01
Net income (loss) attributable to Key common shareholders (in dollars per share)	[3]	(0.32)	0.89	1.94
Income (loss) from continuing operations attributable to Key common shareholders - assuming dilution (in dollars per share)		(0.32)	0.88	1.92
Income (loss) from discontinued operations, net of taxes — assuming dilution (in dollars per share)		0	0	0.01
Net income (loss) attributable to Key common shareholders (in dollars per share)	[3]	$ (0.32)	$ 0.88	$ 1.93

[1] For periods ended in a loss from continuing operations attributable to Key common shareholders, anti-dilutive instruments have been excluded from the calculation of diluted earnings per share.

[2] Assumes conversion of Common Share options and other stock awards and/or convertible preferred stock, as applicable.

[3] EPS may not foot due to rounding.