Condensed Financial Information of the Parent Company - Condensed Statements of Income (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Other income	[1]	$ 146	$ 94	$ 54
Total interest income		4,390	3,319	2,622
Interest on long-term debt with subsidiary trusts		319	218	160
Income (loss) before income taxes and equity in net income (loss) less dividends from subsidiaries		1,981	1,002	1,250
Income tax (expense) benefit		(637)	(179)	(303)
NET INCOME (LOSS)		1,298	790	920
Less: Net income attributable to noncontrolling interests		2	(1)	4
NET INCOME (LOSS) ATTRIBUTABLE TO KEY		1,296	791	916
Key [Member]
Condensed Financial Statements, Captions [Line Items]
Interest income from subsidiaries		10	10	10
Other income		9	11	20
Total interest income		769	696	1,031
Interest on long-term debt with subsidiary trusts		17	14	10
Interest on other borrowed funds		95	69	52
Personnel and other expense		46	101	73
Total expense		158	184	135
Income (loss) before income taxes and equity in net income (loss) less dividends from subsidiaries		611	512	896
Income tax (expense) benefit		29	54	39
Income (loss) before equity in net income (loss) less dividends from subsidiaries		640	566	935
Equity in net income (loss) less dividends from subsidiaries		658	224	(15)
NET INCOME (LOSS)		1,298	790	920
Less: Net income attributable to noncontrolling interests		2	(1)	4
NET INCOME (LOSS) ATTRIBUTABLE TO KEY		1,296	791	916
Key [Member] | Banks [Member]
Condensed Financial Statements, Captions [Line Items]
Bank/Nonbank subsidiaries		750	625	1,000
Key [Member] | Nonbank Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Bank/Nonbank subsidiaries		$ 0	$ 50	$ 1

[1] Net securities gains (losses) totaled less than $1 million for each of the years ended December 31, 2017, 2016, and 2015. For 2017, 2016, and 2015, we did not have any impairment losses related to securities.