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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
EARNINGS
INCOME (LOSS) FROM CONTINUING OPERATIONS		$ 198	$ 236	$ 385	$ 466
Less: Net income (loss) attributable to noncontrolling interests		(1)	1	(1)	3
Income (loss) from continuing operations attributable to Key		199	235	386	463
Income (loss) from continuing operations attributable to Key common shareholders		193	230	375	452
Income (loss) from discontinued operations, net of taxes		3	3	4	8
Net income (loss) attributable to Key common shareholders		$ 196	$ 233	$ 379	$ 460
WEIGHTED-AVERAGE COMMON SHARES
Weighted-average common shares outstanding		831,899	839,454	829,640	843,992
Effect of convertible preferred stock		0	0	0	0
Effect of common share options and other stock awards		6,597	6,858	7,138	7,695
Weighted-average common shares and potential common shares outstanding	[1]	838,496	846,312	836,778	851,687
EARNINGS PER COMMON SHARE
Income (loss) from continuing operations attributable to Key common shareholders		$ 0.23	$ 0.27	$ 0.45	$ 0.53
Income (loss) from discontinued operations, net of taxes					0.01
Net income (loss) attributable to Key common shareholders	[2]	0.23	0.28	0.45	0.54
Income (loss) from continuing operations attributable to Key common shareholders - assuming dilution		0.23	0.27	0.44	0.52
Income (loss) from discontinued operations, net of taxes					0.01
Net income (loss) attributable to Key common shareholders - assuming dilution	[2]	$ 0.23	$ 0.27	$ 0.45	$ 0.53
Series A Preferred Stock [Member]
EARNINGS
Less: Dividends on Series A Preferred Stock		$ 6	$ 5	$ 11	$ 11

[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.