Earnings Per Common Share - Basic and Diluted Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
EARNINGS
Income (loss) from continuing operations		$ 479	$ 407	$ 895	$ 732
Less: Net income (loss) attributable to noncontrolling interests		0	0	0	1
Income (loss) from continuing operations attributable to Key		479	407	895	731
Less: Dividends on Preferred Stock		15	14	29	42
Income (loss) from continuing operations attributable to Key common shareholders		464	393	866	689
Income (loss) from discontinued operations, net of taxes		3	5	5	5
Net income (loss) attributable to Key common shareholders		$ 467	$ 398	$ 871	$ 694
WEIGHTED-AVERAGE COMMON SHARES
Weighted-average common shares outstanding (in shares)		1,052,652	1,076,203	1,054,378	1,083,486
Effect of common share options and other stock awards (in shares)		13,141	16,836	14,561	15,808
Weighted-average common shares and potential common shares outstanding (in shares)	[1]	1,065,793	1,093,039	1,068,939	1,099,294
EARNINGS PER COMMON SHARE
Income (loss) from continuing operations attributable to Key common shareholders (in usd per share)		$ 0.44	$ 0.36	$ 0.82	$ 0.64
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]	0.44	0.37	0.82	0.64
Income (loss) from continuing operations attributable to Key common shareholders - assuming dilution (in usd per share)		0.44	0.36	0.81	0.63
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]	$ 0.44	$ 0.36	$ 0.81	$ 0.63

[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.