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, 2021	Dec. 31, 2020
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Stock	$ 1,257	$ 1,257
Debentures adjustments related to financial instrument hedging	59	70
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	0.942%
KeyCorp Capital II
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 138
Common Stock	4
Principal Amount of of Debentures, Net of Discount	$ 142
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	$ 106
Common Stock	4
Principal Amount of of Debentures, Net of Discount	$ 110
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	$ 20
Common Stock	1
Principal Amount of of Debentures, Net of Discount	$ 21
Interest Rate of Trust Preferred Securities and Debentures	1.55%
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	1.466%
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	1.429%
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	2.325%
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	2.325%
Business Trusts
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 472	483
Common Stock	17	17
Principal Amount of of Debentures, Net of Discount	$ 489	$ 500
Interest Rate of Trust Preferred Securities and Debentures	4.315%	4.464%
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%