Condensed Financial Information of the Parent Company - Condensed Statements of Income (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Other income	[1]	$ 68	$ 176	$ 153
Total interest income		5,235	4,878	4,390
Interest on long-term debt with subsidiary trusts		454	420	319
Income (loss) before income taxes and equity in net income (loss) less dividends from subsidiaries		2,054	2,234	1,981
Income tax (expense) benefit		(314)	(344)	(637)
NET INCOME (LOSS)		1,717	1,866	1,298
Less: Net income attributable to noncontrolling interests		0	0	2
NET INCOME (LOSS) ATTRIBUTABLE TO KEY		1,717	1,866	1,296
Key [Member]
Condensed Financial Statements, Captions [Line Items]
Interest income from subsidiaries		9	11	10
Other income		11	11	9
Total interest income		1,294	1,697	769
Interest on long-term debt with subsidiary trusts		22	20	17
Interest on other borrowed funds		151	137	95
Personnel and other expense		87	69	46
Total expense		260	226	158
Income (loss) before income taxes and equity in net income (loss) less dividends from subsidiaries		1,034	1,471	611
Income tax (expense) benefit		57	55	29
Income (loss) before equity in net income (loss) less dividends from subsidiaries		1,091	1,526	640
Equity in net income (loss) less dividends from subsidiaries		626	340	658
NET INCOME (LOSS)		1,717	1,866	1,298
Less: Net income attributable to noncontrolling interests		0	0	2
NET INCOME (LOSS) ATTRIBUTABLE TO KEY		1,717	1,866	1,296
Key [Member] | Banks [Member]
Condensed Financial Statements, Captions [Line Items]
Bank/Nonbank subsidiaries		1,204	1,675	750
Key [Member] | Nonbank Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Bank/Nonbank subsidiaries		$ 70	$ 0	$ 0

[1] Net securities gains (losses) totaled $20 million for the year ended December 31, 2019, less than $1 million for the year ended December 31, 2018, and $1 million for the year ended December 31, 2017. For 2019, 2018, and 2017, we did not have any impairment losses related to securities.