Condensed Financial Information of the Parent Company - Condensed Statements of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
OPERATING ACTIVITIES
Net income (loss) attributable to Key	$ 1,866	$ 1,296	$ 791
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Deferred income taxes (benefit)	98	303	11
Stock-based compensation expense	99	100	99
Other operating activities, net	14	(601)	175
NET CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES	2,506	1,815	1,689
INVESTING ACTIVITIES
Net decrease (increase) in short-term investments, excluding acquisitions	1,885	(1,672)	(68)
Cash used in acquisitions	0	(144)	(481)
Proceeds from sales, prepayments and maturities of securities available for sale	3,197	3,999	4,241
NET CASH PROVIDED BY (USED IN) INVESTING ACTIVITIES	(2,482)	(2,239)	(6,457)
FINANCING ACTIVITIES
Net proceeds from issuance of long-term debt	2,306	2,852	2,827
Payments on long-term debt	(2,880)	(748)	(1,308)
Repurchase of Treasury Shares	(1,098)	(664)	(140)
Cash dividends paid	(656)	(480)	(335)
NET CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES	(17)	418	4,838
NET INCREASE (DECREASE) IN CASH AND DUE FROM BANKS	7	(6)	70
CASH AND DUE FROM BANKS AT BEGINNING OF YEAR	671	677	607
CASH AND DUE FROM BANKS AT END OF YEAR	678	671	677
Key [Member]
OPERATING ACTIVITIES
Net income (loss) attributable to Key	1,866	1,296	791
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Deferred income taxes (benefit)	109	38	(24)
Stock-based compensation expense	8	11	12
Equity in net (income) loss less dividends from subsidiaries	(340)	(658)	(224)
Other intangible asset amortization	0	0	0
Net (increase) decrease in goodwill and other intangibles	0	0	0
Net (increase) decrease in other assets	(58)	82	(93)
Net increase (decrease) in other liabilities	8	(82)	9
Other operating activities, net	79	(114)	0
NET CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES	1,672	573	471
INVESTING ACTIVITIES
Net decrease (increase) in short-term investments, excluding acquisitions	1	47	(17)
Cash infusion from purchase of Cain Brothers	0	(90)	0
Cash used in acquisitions	0	0	(481)
Proceeds from sales, prepayments and maturities of securities available for sale	0	1	0
Net (increase) decrease in loans to subsidiaries	200	0	160
NET CASH PROVIDED BY (USED IN) INVESTING ACTIVITIES	201	(42)	(338)
FINANCING ACTIVITIES
Net proceeds from issuance of long-term debt	1,250	0	0
Payments on long-term debt	(750)	0	(21)
Repurchase of Treasury Shares	(1,145)	(730)	(140)
Net cash from the issuance (redemption) of Common Shares and preferred stock	412	(350)	1,041
Cash dividends paid	(656)	(480)	(335)
NET CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES	(889)	(1,560)	545
NET INCREASE (DECREASE) IN CASH AND DUE FROM BANKS	984	(1,029)	678
CASH AND DUE FROM BANKS AT BEGINNING OF YEAR	2,257	3,286	2,608
CASH AND DUE FROM BANKS AT END OF YEAR	$ 3,241	$ 2,257	$ 3,286