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				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
EARNINGS
Income (loss) from continuing operations	$ 199		$ 227		$ 401		$ 428
Less: Net income (loss) attributable to noncontrolling interests			5		1		5
Income (loss) from continuing operations attributable to Key	199		222		400		423
Income (loss) from continuing operations attributable to Key common shareholders	193		217		389		412
Income (loss) from discontinued operations, net of taxes	5		14		8		13
Net income (loss) attributable to Key common shareholders	198		231		397		425
WEIGHTED-AVERAGE COMMON SHARES
Weighted-average common shares outstanding	913,736		944,648		917,008		946,995
Effect of dilutive convertible preferred stock, common share options and other stock awards	4,892		3,439		5,311		4,034
Weighted-average common shares and potential common shares outstanding	918,628	[1]	948,087	[1]	922,319	[1]	951,029	[1]
EARNINGS PER COMMON SHARE
Income (loss) from continuing operations attributable to Key common shareholders	$ 0.21		$ 0.23		$ 0.42		$ 0.44
Income (loss) from discontinued operations, net of taxes	$ 0.01		$ 0.01		$ 0.01		$ 0.01
Net income (loss) attributable to Key common shareholders	$ 0.22	[2]	$ 0.24	[2]	$ 0.43	[2]	$ 0.45	[2]
Income (loss) from continuing operations attributable to Key common shareholders - assuming dilution	$ 0.21		$ 0.23		$ 0.42		$ 0.43
Income (loss) from discontinued operations, net of taxes	$ 0.01		$ 0.01		$ 0.01		$ 0.01
Net income (loss) attributable to Key common shareholders - assuming dilution	$ 0.22	[2]	$ 0.24	[2]	$ 0.43	[2]	$ 0.45	[2]
Series A Preferred Stock [Member]
EARNINGS
Less: Dividends on Series A Preferred Stock	$ 6		$ 5		$ 11		$ 11

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

[2] EPS may not foot due to rounding.