Condensed Financial Information of the Parent Company - Condensed Statements of Income (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Other income	[1]	$ 94	$ 54	$ 44
Total interest income		3,319	2,622	2,554
Interest on long-term debt with subsidiary trusts		218	160	133
Income (loss) before income taxes and equity in net income (loss) less dividends from subsidiaries		1,002	1,250	1,296
Income tax (expense) benefit		(179)	(303)	(326)
NET INCOME (LOSS)		790	920	907
Less: Net income attributable to noncontrolling interests		(1)	4	7
NET INCOME (LOSS) ATTRIBUTABLE TO KEY		791	916	900
Key [Member]
Condensed Financial Statements, Captions [Line Items]
Interest income from subsidiaries		10	10	16
Other income		11	20	15
Total interest income		696	1,031	331
Interest on long-term debt with subsidiary trusts		16	10	10
Interest on other borrowed funds		67	52	53
Personnel and other expense		101	73	40
Total expense		184	135	103
Income (loss) before income taxes and equity in net income (loss) less dividends from subsidiaries		512	896	228
Income tax (expense) benefit		54	39	45
Income (loss) before equity in net income (loss) less dividends from subsidiaries		566	935	273
Equity in net income (loss) less dividends from subsidiaries		224	(15)	634
NET INCOME (LOSS)		790	920	907
Less: Net income attributable to noncontrolling interests		(1)	4	7
NET INCOME (LOSS) ATTRIBUTABLE TO KEY		791	916	900
Key [Member] | Banks [Member]
Condensed Financial Statements, Captions [Line Items]
Bank/Nonbank subsidiaries		625	1,000	300
Key [Member] | Nonbank Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Bank/Nonbank subsidiaries		$ 50	$ 1	$ 0

[1] Net securities gains (losses) totaled less than $1 million for the years ended December 31, 2016, 2015, and 2014. For 2016, 2015, and 2014, we did not have any impairment losses related to securities.