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, 2018	Dec. 31, 2017	Dec. 31, 2016
EARNINGS
Income (loss) from continuing operations		$ 1,859	$ 1,291	$ 789
Less: Net income (loss) attributable to noncontrolling interests		0	2	(1)
Income (loss) from continuing operations attributable to Key		1,859	1,289	790
Less: Dividends on preferred stock		66	70	37
Income (loss) from continuing operations attributable to Key common shareholders		1,793	1,219	753
Income (loss) from discontinued operations, net of taxes		7	7	1
Net income (loss) attributable to Key common shareholders		$ 1,800	$ 1,226	$ 754
WEIGHTED-AVERAGE COMMON SHARES
Weighted-average common shares outstanding (in shares)		1,040,890	1,072,078	927,816
Effect of common share options and other stock awards (in shares)		13,792	16,515	10,720
Weighted-average common shares and potential common shares outstanding (000) (in shares)	[1]	1,054,682	1,088,593	938,536
EARNINGS PER COMMON SHARE
Income (loss) from continuing operations attributable to Key common shareholders (in dollars per share)		$ 1.72	$ 1.13	$ 0.81
Income (loss) from discontinued operations, net of taxes (in dollars per share)		0.01	0.01	0
Net income (loss) attributable to Key common shareholders (in dollars per share)	[2]	1.73	1.14	0.81
Income (loss) from continuing operations attributable to Key common shareholders - assuming dilution (in dollars per share)		1.70	1.12	0.80
Income (loss) from discontinued operations, net of taxes (in dollars per share)		0.01	0.01	0
Net income (loss) attributable to Key common shareholders (in dollars per share)	[2]	$ 1.71	$ 1.13	$ 0.80

[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.