Earnings Per Common Share - Basic and Diluted Earnings Per Common Share (Detail) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
EARNINGS
Income (loss) from continuing operations	$ 230		$ 238
Less: Net income (loss) attributable to noncontrolling interests	2
Income (loss) from continuing operations attributable to Key	228		238
Income (loss) from continuing operations attributable to Key common shareholders	222		232
Income (loss) from discontinued operations, net of taxes (a)	5		4
Net income (loss) attributable to Key common shareholders	227		236
WEIGHTED-AVERAGE COMMON SHARES
Weighted-average common shares outstanding	848,580		884,727
Effect of convertible preferred stock	0		0
Effect of common share options and other stock awards	8,542		7,163
Weighted-average common shares and potential common shares outstanding	857,122	[1]	891,890	[1]
EARNINGS PER COMMON SHARE
Income (loss) from continuing operations attributable to Key common shareholders	$ 0.26		$ 0.26
Income (loss) from discontinued operations, net of taxes (a)	$ 0.01
Net income (loss) attributable to Key common shareholders (b)	$ 0.27	[2]	$ 0.27	[2]
Income (loss) from continuing operations attributable to Key common shareholders - assuming dilution	$ 0.26		$ 0.26
Income (loss) from discontinued operations, net of taxes (a)	$ 0.01
Net income (loss) attributable to Key common shareholders - assuming dilution (b)	$ 0.26	[2]	$ 0.26	[2]
Series A Preferred Stock [Member]
EARNINGS
Less: Dividends on Series A Preferred Stock	$ 6		$ 6

[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.