Earnings Per Common Share - Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
EARNINGS
Income (loss) from continuing operations		$ 1,708	$ 1,859	$ 1,291
Less: Net income (loss) attributable to noncontrolling interests		0	0	2
Income (loss) from continuing operations attributable to Key		1,708	1,859	1,289
Less: Dividends on preferred stock		97	66	70
Income (loss) from continuing operations attributable to Key common shareholders		1,611	1,793	1,219
Income (loss) from discontinued operations, net of taxes		9	7	7
Net income (loss) attributable to Key common shareholders		$ 1,620	$ 1,800	$ 1,226
WEIGHTED-AVERAGE COMMON SHARES
Weighted-average common shares outstanding (in shares)		992,091	1,040,890	1,072,078
Effect of common share options and other stock awards (in shares)		10,163	13,792	16,515
Weighted-average common shares and potential common shares outstanding (in shares)	[1]	1,002,254	1,054,682	1,088,593
EARNINGS PER COMMON SHARE
Income (loss) from continuing operations attributable to Key common shareholders (in dollars per share)		$ 1.62	$ 1.72	$ 1.13
Income (loss) from discontinued operations, net of taxes (in dollars per share)		0.01	0.01	0.01
Net income (loss) attributable to Key common shareholders (in dollars per share)	[2]	1.63	1.73	1.14
Income (loss) from continuing operations attributable to Key common shareholders - assuming dilution (in dollars per share)		1.61	1.70	1.12
Income (loss) from discontinued operations, net of taxes (in dollars per share)		0.01	0.01	0.01
Net income (loss) attributable to Key common shareholders (in dollars per share)	[2]	$ 1.62	$ 1.71	$ 1.13

[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.