Earnings Per Common Share - Basic and Diluted Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
EARNINGS
Income (loss) from continuing operations		$ 220	$ 203	$ 686	$ 694
Less: Net income (loss) attributable to noncontrolling interests		(2)		1	6
Income (loss) from continuing operations attributable to Key		222	203	685	688
Income (loss) from continuing operations attributable to Key common shareholders		216	197	668	671
Income (loss) from discontinued operations, net of taxes		(3)	(17)	5	(41)
Net income (loss) attributable to Key common shareholders		$ 213	$ 180	$ 673	$ 630
WEIGHTED-AVERAGE COMMON SHARES
Weighted-average common shares outstanding		831,430	867,350	839,758	875,728
Effect of convertible preferred stock		0	0	0	0
Effect of common share options and other stock awards		7,450	6,772	7,613	6,723
Weighted-average common shares and potential common shares outstanding	[1]	838,880	874,122	847,371	882,451
EARNINGS PER COMMON SHARE
Income (loss) from continuing operations attributable to Key common shareholders		$ 0.26	$ 0.23	$ 0.79	$ 0.77
Income (loss) from discontinued operations, net of taxes			(0.02)	0.01	(0.05)
Net income (loss) attributable to Key common shareholders	[2]	0.26	0.21	0.80	0.72
Income (loss) from continuing operations attributable to Key common shareholders - assuming dilution		0.26	0.23	0.78	0.76
Income (loss) from discontinued operations, net of taxes			(0.02)	0.01	(0.05)
Net income (loss) attributable to Key common shareholders - assuming dilution	[2]	$ 0.25	$ 0.21	$ 0.79	$ 0.71
Series A Preferred Stock [Member]
EARNINGS
Less: Dividends on Series A Preferred Stock		$ 6	$ 6	$ 17	$ 17

[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.