Condensed Financial Information of the Parent Company - Condensed Statements of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
OPERATING ACTIVITIES
Net income (loss) attributable to Key	$ (161)	$ 967	$ 1,917
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Deferred income taxes (benefit)	(351)	(108)	(27)
Stock-based compensation expense	104	121	120
Other operating activities, net	(139)	389	17
NET CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES	664	2,903	4,469
INVESTING ACTIVITIES
Net (increase) decrease in securities available for sale and in short-term and other investments	(6,687)	(8,385)	8,578
Cash used in acquisitions	0	0	(58)
NET CASH PROVIDED BY (USED IN) INVESTING ACTIVITIES	2,497	1,417	(10,934)
FINANCING ACTIVITIES
Net proceeds from issuance of long-term debt	1,646	5,240	16,596
Payments on long-term debt	(9,057)	(5,052)	(8,580)
Net proceeds from Scotiabank investment	2,771	0	0
Cash dividends paid	(927)	(911)	(854)
NET CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES	(2,359)	(4,266)	6,439
NET INCREASE (DECREASE) IN CASH AND DUE FROM BANKS	802	54	(26)
CASH AND DUE FROM BANKS AT BEGINNING OF YEAR	941	887	913
CASH AND DUE FROM BANKS AT END OF YEAR	1,743	941	887
Key
OPERATING ACTIVITIES
Net income (loss) attributable to Key	(161)	967	1,917
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Deferred income taxes (benefit)	10	(6)	6
Stock-based compensation expense	10	9	117
Equity in net (income) loss less dividends from subsidiaries	588	(575)	(1,521)
Net (increase) decrease in accrued income and other assets	(91)	44	23
Net increase (decrease) in accrued expenses and other liabilities	25	3	(24)
Other operating activities, net	(706)	122	(480)
NET CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES	(325)	564	38
INVESTING ACTIVITIES
Net (increase) decrease in securities available for sale and in short-term and other investments	(19)	(14)	(26)
Cash used in acquisitions	0	0	0
Advances to subsidiaries	(250)	0	0
Sale or repayments of advances to subsidiaries	200	16	(200)
NET CASH PROVIDED BY (USED IN) INVESTING ACTIVITIES	(69)	2	(226)
FINANCING ACTIVITIES
Net proceeds from issuance of long-term debt	1,000	0	1,350
Payments on long-term debt	0	0	0
Repurchase of Treasury Shares	(28)	(73)	(44)
Net cash from the issuance (redemption) of Common Shares and preferred stock	0	0	590
Net proceeds from Scotiabank investment	2,771	0	0
Cash dividends paid	(927)	(912)	(855)
NET CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES	2,816	(985)	1,041
NET INCREASE (DECREASE) IN CASH AND DUE FROM BANKS	2,422	(419)	853
CASH AND DUE FROM BANKS AT BEGINNING OF YEAR	2,727	3,146	2,293
CASH AND DUE FROM BANKS AT END OF YEAR	$ 5,149	$ 2,727	$ 3,146