Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
EARNINGS
Income (loss) from continuing operations		$ 273	$ 286	$ 492	$ 597
Less: Net income (loss) attributable to noncontrolling interests		0	0	0	0
Income (loss) from continuing operations attributable to Key		273	286	492	597
Less: Dividends on Preferred Stock		36	36	72	72
Income (loss) from continuing operations attributable to Key common shareholders		237	250	420	525
Income (loss) from discontinued operations, net of taxes		1	1	1	2
Net income (loss) attributable to Key common shareholders		$ 238	$ 251	$ 421	$ 527
WEIGHTED-AVERAGE COMMON SHARES
Weighted-average Common Shares outstanding (in shares)		931,726	926,741	930,776	926,807
Effect of Common Share options and other stock awards (in shares)		6,761	3,713	7,040	5,513
Weighted-average Common Shares and potential Common Shares outstanding (in shares)	[1]	938,487	930,454	937,816	932,320
EARNINGS PER COMMON SHARE
Income (loss) from continuing operations attributable to Key common shareholders (in usd per share)		$ 0.25	$ 0.27	$ 0.45	$ 0.57
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]	0.25	0.27	0.45	0.57
Income (loss) from continuing operations attributable to Key common shareholders - assuming dilution (in usd per share)		0.25	0.27	0.45	0.56
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]	$ 0.25	$ 0.27	$ 0.45	$ 0.57

[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.