Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
EARNINGS
Income (loss) from continuing operations		$ 490	$ (411)	$ 1,319	$ 81
Less: Dividends on Preferred Stock		36	36	108	108
Income (loss) from continuing operations attributable to Key common shareholders		454	(447)	1,211	(27)
Income (loss) from discontinued operations, net of taxes		(1)	1	0	2
Net income (loss) attributable to Key common shareholders		$ 453	$ (446)	$ 1,211	$ (25)
WEIGHTED-AVERAGE COMMON SHARES
Weighted-average Common Shares outstanding (in shares)		1,100,830	948,979	1,099,520	936,962
Effect of Common Share options and other stock awards (in shares)		9,845	0	8,864	0
Weighted-average Common Shares and potential Common Shares outstanding (in shares)	[1]	1,110,675	948,979	1,108,384	936,962
EARNINGS PER COMMON SHARE
Income (loss) from continuing operations attributable to Key common shareholders (in usd per share)		$ 0.41	$ (0.47)	$ 1.10	$ (0.03)
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]	0.41	(0.47)	1.10	(0.03)
Income (loss) from continuing operations attributable to Key common shareholders - assuming dilution (in usd per share)		0.41	(0.47)	1.09	(0.03)
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]	$ 0.41	$ (0.47)	$ 1.09	$ (0.03)

[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