Earnings Per Common Share - Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
EARNINGS
Income (loss) from continuing operations		$ 406	$ 416
Less: Net income (loss) attributable to noncontrolling interests		0	0
Income (loss) from continuing operations attributable to Key		406	416
Less: Dividends on Preferred Stock		20	14
Income (loss) from continuing operations attributable to Key common shareholders		386	402
Income (loss) from discontinued operations, net of taxes		1	2
Net income (loss) attributable to Key common shareholders		$ 387	$ 404
WEIGHTED-AVERAGE COMMON SHARES
Weighted-average common shares outstanding (000) (in shares)		1,006,717	1,056,037
Effect of common share options and other stock awards (in shares)		9,787	15,749
Weighted-average common shares and potential common shares outstanding (in shares)	[1]	1,016,504	1,071,786
EARNINGS PER COMMON SHARE
Income (loss) from continuing operations attributable to Key common shareholders (in usd per share)		$ 0.38	$ 0.38
Income (loss) from discontinued operations, net of taxes (in usd per share)		0	0
Net income (loss) attributable to Key common shareholders (in usd per share)	[2]	0.38	0.38
Income (loss) from continuing operations attributable to Key common shareholders - assuming dilution (in usd per share)		0.38	0.38
Income (loss) from discontinued operations, net of taxes - assuming dilution (in usd per share)		0	0
Net income (loss) attributable to Key common shareholders (in usd per share)	[2]	$ 0.38	$ 0.38

[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.