Trust Preferred Securities Issued by Unconsolidated Subsidiaries - Summary of Trust Preferred Securities, Common Stock and Related Debentures (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
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	0.995%
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	112
Common Stock	4
Principal Amount of Debentures, Net of Discount	116
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	146
Common Stock	4
Principal Amount of Debentures, Net of Discount	150
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	414	408	391
Common Stock	14	14	14
Principal Amount of Debentures, Net of Discount	$ 428	$ 422	$ 405
Interest Rate of Trust Preferred Securities and Debentures	4.968%	4.926%	4.823%