Condensed Financial Information of the Parent Company - Condensed Statements of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
OPERATING ACTIVITIES
Net income (loss) attributable to Key	$ 1,717	$ 1,866	$ 1,296
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Deferred income taxes (benefit)	53	98	303
Stock-based compensation expense	96	99	100
Other operating activities, net	560	14	(601)
NET CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES	2,906	2,506	1,815
INVESTING ACTIVITIES
Net decrease (increase) in short-term investments, excluding acquisitions	1,290	1,885	(1,672)
Proceeds from sales, prepayments and maturities of securities available for sale	3,586	3,197	3,999
NET CASH PROVIDED BY (USED IN) INVESTING ACTIVITIES	(4,918)	(2,482)	(2,239)
FINANCING ACTIVITIES
Net proceeds from issuance of long-term debt	2,129	2,306	2,852
Payments on long-term debt	(3,634)	(2,880)	(748)
Repurchase of Treasury Shares	(835)	(1,098)	(664)
Cash dividends paid	(804)	(656)	(480)
NET CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES	2,066	(17)	418
NET INCREASE (DECREASE) IN CASH AND DUE FROM BANKS	54	7	(6)
CASH AND DUE FROM BANKS AT BEGINNING OF YEAR	678	671	677
CASH AND DUE FROM BANKS AT END OF YEAR	732	678	671
Key [Member]
OPERATING ACTIVITIES
Net income (loss) attributable to Key	1,717	1,866	1,296
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Deferred income taxes (benefit)	(43)	109	38
Stock-based compensation expense	8	8	11
Equity in net (income) loss less dividends from subsidiaries	(626)	(340)	(658)
Net (increase) decrease in other assets	39	(58)	82
Net increase (decrease) in other liabilities	11	8	(82)
Other operating activities, net	244	79	(114)
NET CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES	1,350	1,672	573
INVESTING ACTIVITIES
Net decrease (increase) in short-term investments, excluding acquisitions	(6)	1	47
Cash infusion from purchase of Cain Brothers	0	0	(90)
Proceeds from sales, prepayments and maturities of securities available for sale	0	0	1
Net (increase) decrease in loans to subsidiaries	15	200	0
NET CASH PROVIDED BY (USED IN) INVESTING ACTIVITIES	9	201	(42)
FINANCING ACTIVITIES
Net proceeds from issuance of long-term debt	750	1,250	0
Payments on long-term debt	(300)	(750)	0
Repurchase of Treasury Shares	(868)	(1,145)	(730)
Net cash from the issuance (redemption) of Common Shares and preferred stock	435	412	(350)
Cash dividends paid	(804)	(656)	(480)
NET CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES	(787)	(889)	(1,560)
NET INCREASE (DECREASE) IN CASH AND DUE FROM BANKS	572	984	(1,029)
CASH AND DUE FROM BANKS AT BEGINNING OF YEAR	3,241	2,257	3,286
CASH AND DUE FROM BANKS AT END OF YEAR	$ 3,813	$ 3,241	$ 2,257