Trust Preferred Securities Issued by Unconsolidated Subsidiaries (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Stock	$ 1,257	$ 1,257
Debentures adjustments related to financial instrument hedging	11	15
Total
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	427	431
Common Stock	17	17
Principal Amount of Debentures, Net of Discount	$ 444	$ 448
Interest Rate of Trust Preferred Securities and Debentures	6.979%	6.981%
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 Debentures, Net of Discount	$ 162
Interest Rate of Trust Preferred Securities and Debentures	6.396%
KeyCorp Capital II
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 85
Common Stock	4
Principal Amount of Debentures, Net of Discount	$ 89
Interest Rate of Trust Preferred Securities and Debentures	6.875%
KeyCorp Capital II | Treasury Rate
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Basis spread on variable rate	0.20%
KeyCorp Capital III
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	7.75%
KeyCorp Capital III | Treasury Rate
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Basis spread on variable rate	0.25%
HNC Statutory Trust III
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 21
Common Stock	1
Principal Amount of Debentures, Net of Discount	$ 22
Interest Rate of Trust Preferred Securities and Debentures	7.04%
HNC Statutory Trust IV
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 18
Common Stock	1
Principal Amount of Debentures, Net of Discount	$ 19
Interest Rate of Trust Preferred Securities and Debentures	6.932%
Willow Grove Statutory Trust I
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 20
Common Stock	1
Principal Amount of Debentures, Net of Discount	$ 21
Interest Rate of Trust Preferred Securities and Debentures	6.956%
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 Debentures, Net of Discount	$ 8
Interest Rate of Trust Preferred Securities and Debentures	7.822%
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 Debentures, Net of Discount	$ 8
Interest Rate of Trust Preferred Securities and Debentures	7.822%
Keycorp Capital II and III | Redemption Upon Either Tax or a Capital Treatment Event | Treasury Rate
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Basis spread on variable rate	0.50%
Westbank Capital Trust II and III | SOFR
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Basis spread on variable rate	0.26161%