Earnings Per Common Share - Basic and Diluted Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
EARNINGS
Income (loss) from continuing operations		$ 482	$ 363	$ 1,377	$ 1,095
Less: Net income (loss) attributable to noncontrolling interests		0	0	0	1
Income (loss) from continuing operations attributable to Key		482	363	1,377	1,094
Less: Dividends on Preferred Stock		14	14	43	56
Income (loss) from continuing operations attributable to Key common shareholders		468	349	1,334	1,038
Income (loss) from discontinued operations, net of taxes		0	1	5	6
Net income (loss) attributable to Key common shareholders		$ 468	$ 350	$ 1,339	$ 1,044
WEIGHTED-AVERAGE COMMON SHARES
Weighted-average common shares outstanding (in shares)		1,036,479	1,073,390	1,048,397	1,075,296
Effect of common share options and other stock awards (in shares)		13,497	15,451	14,419	16,359
Weighted-average common shares and potential common shares outstanding (in shares)	[1]	1,049,976	1,088,841	1,062,816	1,091,655
EARNINGS PER COMMON SHARE
Income (loss) from continuing operations attributable to Key common shareholders (in usd per share)		$ 0.45	$ 0.32	$ 1.28	$ 0.96
Income (loss) from discontinued operations, net of taxes (in usd per share)		0	0	0.01	0.01
Net income (loss) attributable to Key common shareholders (in usd per share)	[2]	0.45	0.32	1.27	0.97
Income (loss) from continuing operations attributable to Key common shareholders - assuming dilution (in usd per share)		0.45	0.32	1.26	0.95
Income (loss) from discontinued operations, net of taxes - assuming dilution (in usd per share)		0	0	0.01	0.01
Net income (loss) attributable to Key common shareholders (in usd per share)	[2]	$ 0.45	$ 0.32	$ 1.26	$ 0.96

[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.