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				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
EARNINGS
Income (loss) from continuing operations	$ 234		$ 218		$ 635		$ 646
Less: Net income (loss) attributable to noncontrolling interests	(1)		2				7
Income (loss) from continuing operations attributable to Key	235		216		635		639
Income (loss) from continuing operations attributable to Key common shareholders	229		211		618		623
Income (loss) from discontinued operations, net of taxes	37		3		45		16
Net income (loss) attributable to Key common shareholders	266		214		663		639
WEIGHTED-AVERAGE COMMON SHARES
Weighted-average common shares outstanding	901,904		936,223		911,918		943,378
Effect of dilutive convertible preferred stock, common share options and other stock awards	26,950		4,541		5,661		4,204
Weighted-average common shares and potential common shares outstanding	928,854	[1]	940,764	[1]	917,579	[1]	947,582	[1]
EARNINGS PER COMMON SHARE
Income (loss) from continuing operations attributable to Key common shareholders	$ 0.25		$ 0.23		$ 0.68		$ 0.66
Income (loss) from discontinued operations, net of taxes	$ 0.04				$ 0.05		$ 0.02
Net income (loss) attributable to Key common shareholders	$ 0.29	[2]	$ 0.23	[2]	$ 0.73	[2]	$ 0.68	[2]
Income (loss) from continuing operations attributable to Key common shareholders - assuming dilution	$ 0.25		$ 0.22		$ 0.67		$ 0.66
Income (loss) from discontinued operations, net of taxes	$ 0.04				$ 0.05		$ 0.02
Net income (loss) attributable to Key common shareholders - assuming dilution	$ 0.29	[2]	$ 0.23	[2]	$ 0.72	[2]	$ 0.67	[2]
Series A Preferred Stock [Member]
EARNINGS
Less: Dividends on Series A Preferred Stock	$ 6		$ 5		$ 17		$ 16

[1] Assumes conversion of stock options and/or Series A Preferred Stock, as applicable.

[2] EPS may not foot due to rounding.