Condensed Financial Information of the Parent Company - Condensed Statements of Income (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Other income	[1]	$ 176	$ 153	$ 114
Total interest income		4,878	4,390	3,319
Interest on long-term debt with subsidiary trusts		420	319	218
Income (loss) before income taxes and equity in net income (loss) less dividends from subsidiaries		2,234	1,981	1,002
Income tax (expense) benefit		(344)	(637)	(179)
NET INCOME (LOSS)		1,866	1,298	790
Less: Net income attributable to noncontrolling interests		0	2	(1)
NET INCOME (LOSS) ATTRIBUTABLE TO KEY		1,866	1,296	791
Key [Member]
Condensed Financial Statements, Captions [Line Items]
Interest income from subsidiaries		11	10	10
Other income		11	9	11
Total interest income		1,697	769	696
Interest on long-term debt with subsidiary trusts		20	17	14
Interest on other borrowed funds		137	95	69
Personnel and other expense		69	46	101
Total expense		226	158	184
Income (loss) before income taxes and equity in net income (loss) less dividends from subsidiaries		1,471	611	512
Income tax (expense) benefit		55	29	54
Income (loss) before equity in net income (loss) less dividends from subsidiaries		1,526	640	566
Equity in net income (loss) less dividends from subsidiaries		340	658	224
NET INCOME (LOSS)		1,866	1,298	790
Less: Net income attributable to noncontrolling interests		0	2	(1)
NET INCOME (LOSS) ATTRIBUTABLE TO KEY		1,866	1,296	791
Key [Member] | Banks [Member]
Condensed Financial Statements, Captions [Line Items]
Bank/Nonbank subsidiaries		1,675	750	625
Key [Member] | Nonbank Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Bank/Nonbank subsidiaries		$ 0	$ 0	$ 50

[1] Net securities gains (losses) totaled less than $1 million for each of the years ended December 31, 2018, 2017, and 2016. For 2018, 2017, and 2016, we did not have any impairment losses related to securities.