Trust Preferred Securities Issued by Unconsolidated Subsidiaries - Summary of Trust Preferred Securities, Common Stock and Related Debentures (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Stock	$ 1,257	$ 1,257
Debentures adjustments related to financial instrument hedging	78	57
KeyCorp Capital I
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	156
Common Stock	6
Principal Amount of of Debentures, Net of Discount	$ 162
Interest Rate of Trust Preferred Securities and Debentures	2.173%
KeyCorp Capital II
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 114
Common Stock	4
Principal Amount of of Debentures, Net of Discount	$ 118
Interest Rate of Trust Preferred Securities and Debentures	6.875%
KeyCorp Capital III
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 149
Common Stock	4
Principal Amount of of Debentures, Net of Discount	$ 153
Interest Rate of Trust Preferred Securities and Debentures	7.75%
HNC Statutory Trust III
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 19
Common Stock	1
Principal Amount of of Debentures, Net of Discount	$ 20
Interest Rate of Trust Preferred Securities and Debentures	1.76%
Willow Grove Statutory Trust I
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 19
Common Stock	1
Principal Amount of of Debentures, Net of Discount	$ 20
Interest Rate of Trust Preferred Securities and Debentures	1.631%
HNC Statutory Trust IV
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 17
Common Stock	1
Principal Amount of of Debentures, Net of Discount	$ 18
Interest Rate of Trust Preferred Securities and Debentures	2.04%
Westbank Capital Trust II
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 8
Common Stock	0
Principal Amount of of Debentures, Net of Discount	$ 8
Interest Rate of Trust Preferred Securities and Debentures	2.496%
Westbank Capital Trust III
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 8
Common Stock	0
Principal Amount of of Debentures, Net of Discount	$ 8
Interest Rate of Trust Preferred Securities and Debentures	2.496%
Business Trusts
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 490	466
Common Stock	17	17
Principal Amount of of Debentures, Net of Discount	$ 507	$ 483
Interest Rate of Trust Preferred Securities and Debentures	4.932%	5.214%
Treasury Rate | KeyCorp Capital II
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Basis spread on variable rate	0.20%
Treasury Rate | KeyCorp Capital III
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Basis spread on variable rate	0.25%
London Interbank Offered Rate (LIBOR) | KeyCorp Capital I
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Basis spread on variable rate	0.74%
London Interbank Offered Rate (LIBOR) | HNC Statutory Trust III
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Basis spread on variable rate	1.40%
London Interbank Offered Rate (LIBOR) | Willow Grove Statutory Trust I
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Basis spread on variable rate	1.31%
London Interbank Offered Rate (LIBOR) | HNC Statutory Trust IV
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Basis spread on variable rate	1.28%
London Interbank Offered Rate (LIBOR) | Westbank Capital Trust II and III
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 | Treasury Rate | Keycorp Capital II and III
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Basis spread on variable rate	0.50%