Trust Preferred Securities Issued by Unconsolidated Subsidiaries - Summary of Trust Preferred Securities, Common Stock and Related Debentures (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 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	1.024%
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	$ 111
Common Stock	4
Principal Amount of Debentures, Net of Discount	$ 115
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	$ 145
Common Stock	4
Principal Amount of Debentures, Net of Discount	$ 149
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	$ 412	408	397
Common Stock	14	14	14
Principal Amount of Debentures, Net of Discount	$ 426	$ 422	$ 411
Interest Rate of Trust Preferred Securities and Debentures	4.968%	4.926%	4.856%