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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
EARNINGS
Income (loss) from continuing operations		$ 407	$ 198	$ 732	$ 385
Less: Net income (loss) attributable to noncontrolling interests		0	(1)	1	(1)
Income (loss) from continuing operations attributable to Key		407	199	731	386
Less: Dividends on Preferred Stock		14	6	42	11
Income (loss) from continuing operations attributable to Key common shareholders		393	193	689	375
Income (loss) from discontinued operations, net of taxes		5	3	5	4
Net income (loss) attributable to Key common shareholders		$ 398	$ 196	$ 694	$ 379
WEIGHTED-AVERAGE COMMON SHARES
Weighted-average common shares outstanding (in shares)		1,076,203	831,899	1,083,486	829,640
Effect of convertible preferred stock (in shares)		0	0	0	0
Effect of common share options and other stock awards (in shares)		16,836	6,597	15,808	7,138
Weighted-average common shares and potential common shares outstanding (in shares)	[1]	1,093,039	838,496	1,099,294	836,778
EARNINGS PER COMMON SHARE
Income (loss) from continuing operations attributable to Key common shareholders (in usd per share)		$ 0.36	$ 0.23	$ 0.64	$ 0.45
Income (loss) from discontinued operations, net of taxes (in usd per share)		0.00	0.00	0.00	0.00
Net income (loss) attributable to Key common shareholders (in usd per share)	[2]	0.37	0.23	0.64	0.45
Income (loss) from continuing operations attributable to Key common shareholders - assuming dilution (in usd per share)		0.36	0.23	0.63	0.44
Income (loss) from discontinued operations, net of taxes (in usd per share)		0.00	0.00	0.00	0.00
Net income (loss) attributable to Key common shareholders (in usd per share)	[2]	$ 0.36	$ 0.23	$ 0.63	$ 0.45

[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.