Trust Preferred Securities Issued by Unconsolidated Subsidiaries - Summary of Trust Preferred Securities, Common Stock and Related Debentures (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Stock	$ 1,017	$ 1,017	$ 1,017
KeyCorp Capital I [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	156
Common Stock	6
Principal Amount of Debentures, Net of Discount	162
Interest Rate of Trust Preferred Securities and Debentures	1.014%
Maturity of Trust Preferred Securities and Debentures	2028
KeyCorp Capital II [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	102
Common Stock	4
Principal Amount of Debentures, Net of Discount	106
Interest Rate of Trust Preferred Securities and Debentures	6.875%
Maturity of Trust Preferred Securities and Debentures	2029
KeyCorp Capital III [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	133
Common Stock	4
Principal Amount of Debentures, Net of Discount	137
Interest Rate of Trust Preferred Securities and Debentures	7.75%
Maturity of Trust Preferred Securities and Debentures	2029
Business Trusts [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	391	417	423
Common Stock	14	14	14
Principal Amount of Debentures, Net of Discount	$ 405	$ 431	$ 437
Interest Rate of Trust Preferred Securities and Debentures	4.834%	5.025%	5.095%