Condensed Financial Information of the Parent Company - Condensed Statements of Cash Flows (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
OPERATING ACTIVITIES
Net income (loss) attributable to Key	$ 791	$ 916	$ 900
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Deferred income taxes (benefit)	11	(76)	5
Stock-based compensation expense	99	58	44
Other operating activities, net	195	(151)	384
NET CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES	1,689	1,131	1,320
INVESTING ACTIVITIES
Net decrease (increase) in short-term investments, excluding acquisitions	(68)	1,562	1,358
Purchases of securities available for sale	(5,718)	(4,090)	(3,797)
Cash used in acquisitions	(481)	0	(114)
Proceeds from sales, prepayments and maturities of securities available for sale	4,241	3,098	2,860
NET CASH PROVIDED BY (USED IN) INVESTING ACTIVITIES	(6,457)	(2,023)	(2,816)
FINANCING ACTIVITIES
Net proceeds from issuance of long-term debt	2,827	3,756	1,727
Payments on long-term debt	(1,308)	(1,172)	(1,355)
Repurchase of Treasury Shares	(140)	(448)	(484)
Cash dividends paid	(335)	(267)	(240)
NET CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES	4,838	846	1,532
NET INCREASE (DECREASE) IN CASH AND DUE FROM BANKS	70	(46)	36
CASH AND DUE FROM BANKS AT BEGINNING OF YEAR	607	653	617
CASH AND DUE FROM BANKS AT END OF YEAR	677	607	653
Key [Member]
OPERATING ACTIVITIES
Net income (loss) attributable to Key	791	916	900
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Deferred income taxes (benefit)	(24)	10	(8)
Stock-based compensation expense	12	9	14
Equity in net (income) loss less dividends from subsidiaries	(224)	15	(634)
Other intangible asset amortization	0	2	0
Net (increase) decrease in goodwill and other intangibles	0	86	0
Net (increase) decrease in other assets	(93)	29	(53)
Net increase (decrease) in other liabilities	9	(7)	98
Other operating activities, net	0	(52)	24
NET CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES	471	1,008	341
INVESTING ACTIVITIES
Net decrease (increase) in short-term investments, excluding acquisitions	(17)	(27)	4
Purchases of securities available for sale	0	(11)	(2)
Cash used in acquisitions	(481)	0	(114)
Proceeds from sales, prepayments and maturities of securities available for sale	0	20	0
Net (increase) decrease in loans to subsidiaries	160	(146)	257
NET CASH PROVIDED BY (USED IN) INVESTING ACTIVITIES	(338)	(164)	145
FINANCING ACTIVITIES
Net proceeds from issuance of long-term debt	0	1,000	0
Payments on long-term debt	(21)	(750)	0
Repurchase of Treasury Shares	(140)	(448)	(484)
Net proceeds from the issuance of common shares and preferred stock	1,041	22	27
Cash dividends paid	(335)	(267)	(240)
NET CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES	545	(443)	(697)
NET INCREASE (DECREASE) IN CASH AND DUE FROM BANKS	678	401	(211)
CASH AND DUE FROM BANKS AT BEGINNING OF YEAR	2,608	2,207	2,418
CASH AND DUE FROM BANKS AT END OF YEAR	$ 3,286	$ 2,608	$ 2,207