Condensed Financial Information of the Parent Company - Condensed Balance Sheets (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012	Dec. 31, 2011
Condensed Financial Statements, Captions [Line Items]
Cash and due from banks	$ 653		$ 617		$ 584	$ 693
Short-term investments	4,269		5,590
Securities available for sale	13,360		12,346
Other investments	760		969
Banks	56,587		53,609
Goodwill	1,057		979		979
Other intangible assets	101		127
Corporate owned life insurance	3,479		3,408
Derivative assets	609		407
Accrued income and other assets	2,952	[1]	3,015	[1]
Total assets	93,821		92,934
Accrued expense and other liabilities	1,621		1,557
Derivative liabilities	784		414
Total long-term debt	7,875		7,650
Total liabilities	83,279		82,614
SHAREHOLDERS' EQUITY	10,530		10,303
Total liabilities and equity	93,821		92,934
Key [Member]
Condensed Financial Statements, Captions [Line Items]
Cash and due from banks	2,207		2,418		75	2,031
Short-term investments	31		35
Securities available for sale	22		20
Other investments	15		15
Banks	301		558
Total investment in subsidiaries	10,630		9,921
Goodwill	244		167
Other intangible assets	11
Corporate owned life insurance	212		204
Derivative assets	11		6
Accrued income and other assets	356		333
Total assets	14,040		13,677
Accrued expense and other liabilities	511		413
Derivative liabilities	2		10
Total long-term debt	422		398
Total long-term debt	2,997		2,951
Total liabilities	3,510		3,374
SHAREHOLDERS' EQUITY	10,530		10,303
Total liabilities and equity	14,040		13,677
Key [Member] | Banks [Member]
Condensed Financial Statements, Captions [Line Items]
Banks	90		90
Banks	9,998		9,342
Key [Member] | Nonbank Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Banks	211		468
Banks	632		579
Key [Member] | Unaffiliated Companies [Member]
Condensed Financial Statements, Captions [Line Items]
Total long-term debt	$ 2,575		$ 2,553

[1] The assets of the VIEs can only be used by the particular VIE, and there is no recourse to Key with respect to the liabilities of the consolidated LIHTC VIEs.