Trust Preferred Securities Issued by Unconsolidated Subsidiaries - Summary of Trust Preferred Securities, Common Stock and Related Debentures (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Stock	$ 1,257	$ 1,257
Debentures adjustments related to financial instrument hedging	40	55
KeyCorp Capital I [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	155
Common Stock	6
Principal Amount of of Debentures, Net of Discount	$ 161
Interest Rate of Trust Preferred Securities and Debentures	3.077%
KeyCorp Capital II [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 99
Common Stock	4
Principal Amount of of Debentures, Net of Discount	$ 103
Interest Rate of Trust Preferred Securities and Debentures	6.875%
KeyCorp Capital III [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 128
Common Stock	4
Principal Amount of of Debentures, Net of Discount	$ 132
Interest Rate of Trust Preferred Securities and Debentures	7.75%
HNC Statutory Trust III [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 18
Common Stock	1
Principal Amount of of Debentures, Net of Discount	$ 19
Interest Rate of Trust Preferred Securities and Debentures	3.71%
Willow Grove Statutory Trust I [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 18
Common Stock	1
Principal Amount of of Debentures, Net of Discount	$ 19
Interest Rate of Trust Preferred Securities and Debentures	3.644%
HNC Statutory Trust IV [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 16
Common Stock	1
Principal Amount of of Debentures, Net of Discount	$ 17
Interest Rate of Trust Preferred Securities and Debentures	3.619%
Westbank Capital Trust II [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 7
Common Stock	0
Principal Amount of of Debentures, Net of Discount	$ 7
Interest Rate of Trust Preferred Securities and Debentures	4.528%
Westbank Capital Trust III [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 7
Common Stock	0
Principal Amount of of Debentures, Net of Discount	$ 7
Interest Rate of Trust Preferred Securities and Debentures	4.528%
Business Trusts [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 448	463
Common Stock	17	17
Principal Amount of of Debentures, Net of Discount	$ 465	$ 480
Interest Rate of Trust Preferred Securities and Debentures	5.364%	4.977%
Treasury Rate [Member] | KeyCorp Capital II [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Basis spread on variable rate	0.20%
Treasury Rate [Member] | KeyCorp Capital III [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Basis spread on variable rate	0.25%
London Interbank Offered Rate (LIBOR) [Member] | KeyCorp Capital I [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Basis spread on variable rate	0.74%
London Interbank Offered Rate (LIBOR) [Member] | HNC Statutory Trust III [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Basis spread on variable rate	1.40%
London Interbank Offered Rate (LIBOR) [Member] | Willow Grove Statutory Trust I [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Basis spread on variable rate	1.31%
London Interbank Offered Rate (LIBOR) [Member] | HNC Statutory Trust IV [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Basis spread on variable rate	1.28%
London Interbank Offered Rate (LIBOR) [Member] | Westbank Capital Trust II and III [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Basis spread on variable rate	2.19%
Redemption upon either tax or a capital treatment event [Member] | Treasury Rate [Member] | Keycorp Capital II and III [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Basis spread on variable rate	0.50%