Condensed Financial Information of the Parent Company - Condensed Statements of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
OPERATING ACTIVITIES
Net income (loss) attributable to Key	$ 2,625	$ 1,343	$ 1,717
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Deferred income taxes (benefit)	146	(191)	53
Stock-based compensation expense	104	101	96
Other operating activities, net	(615)	(893)	560
NET CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES	1,153	1,673	2,906
INVESTING ACTIVITIES
Net decrease (increase) in short-term investments, excluding acquisitions	5,184	(14,922)	1,290
Proceeds from sales, prepayments and maturities of securities available for sale	7,623	9,923	3,586
NET CASH PROVIDED BY (USED IN) INVESTING ACTIVITIES	(15,068)	(24,721)	(4,918)
FINANCING ACTIVITIES
Net proceeds from issuance of long-term debt	1,203	3,607	2,129
Payments on long-term debt	(2,566)	(2,508)	(3,634)
Repurchase of Treasury Shares	(559)	(134)	(835)
Cash dividends paid	(823)	(829)	(804)
NET CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES	13,737	23,407	2,066
NET INCREASE (DECREASE) IN CASH AND DUE FROM BANKS	(178)	359	54
CASH AND DUE FROM BANKS AT BEGINNING OF YEAR	1,091	732	678
CASH AND DUE FROM BANKS AT END OF YEAR	913	1,091	732
Key
OPERATING ACTIVITIES
Net income (loss) attributable to Key	2,625	1,343	1,717
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Deferred income taxes (benefit)	(22)	2	(43)
Stock-based compensation expense	9	11	8
Equity in net (income) loss less dividends from subsidiaries	(754)	(238)	(626)
Net (increase) decrease in other assets	13	(66)	39
Net increase (decrease) in other liabilities	48	12	11
Other operating activities, net	(414)	131	244
NET CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES	1,505	1,195	1,350
INVESTING ACTIVITIES
Net decrease (increase) in short-term investments, excluding acquisitions	(15)	(7)	(6)
Proceeds from sales, prepayments and maturities of securities available for sale	0	0	0
Net (increase) decrease in loans to subsidiaries	0	0	15
NET CASH PROVIDED BY (USED IN) INVESTING ACTIVITIES	(15)	(7)	9
FINANCING ACTIVITIES
Net proceeds from issuance of long-term debt	0	800	750
Payments on long-term debt	(997)	(1,003)	(300)
Repurchase of Treasury Shares	(1,176)	(170)	(868)
Net cash from the issuance (redemption) of Common Shares and preferred stock	0	0	435
Cash dividends paid	(823)	(829)	(804)
NET CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES	(2,996)	(1,202)	(787)
NET INCREASE (DECREASE) IN CASH AND DUE FROM BANKS	(1,506)	(14)	572
CASH AND DUE FROM BANKS AT BEGINNING OF YEAR	3,799	3,813	3,241
CASH AND DUE FROM BANKS AT END OF YEAR	$ 2,293	$ 3,799	$ 3,813