Trust Preferred Securities Issued by Unconsolidated Subsidiaries - Summary of Trust Preferred Securities, Common Stock and Related Debentures (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust preferred securities redeemed	$ 21
Common Stock	1,257	$ 1,017
Debentures adjustments related to financial instrument hedging	59	68
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 of Debentures, Net of Discount	$ 162
Interest Rate of Trust Preferred Securities and Debentures	1.586%
KeyCorp Capital II [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 107
Common Stock	4
Principal Amount of of Debentures, Net of Discount	$ 111
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	$ 137
Common Stock	4
Principal Amount of of Debentures, Net of Discount	$ 141
Interest Rate of Trust Preferred Securities and Debentures	7.75%
Harleysville Statutory Trust I [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 5
Common Stock	0
Principal Amount of of Debentures, Net of Discount	$ 5
Interest Rate of Trust Preferred Securities and Debentures	10.20%
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	2.32%
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	2.273%
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	2.167%
Alliance Capital Trust II [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 4
Common Stock	0
Principal Amount of of Debentures, Net of Discount	$ 4
Interest Rate of Trust Preferred Securities and Debentures	10.875%
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	3.187%
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	3.187%
Business Trusts [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust Preferred Securities, Net of Discount	$ 475	408
Common Stock	17	14
Principal Amount of of Debentures, Net of Discount	$ 492	$ 422
Interest Rate of Trust Preferred Securities and Debentures	4.845%	4.961%
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%
Debt Instrument, Redemption, Period Five [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%
Debt Instrument, Redemption, Period One [Member] | Harleysville Statutory Trust I [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Debt instrument, redemption price, percentage	105.10%
Debt Instrument, Redemption, Period Two [Member] | Harleysville Statutory Trust I [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Debt instrument, redemption price, percentage	100.00%
Debt Instrument, Redemption, Period Three [Member] | Alliance Capital Trust II [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Debt instrument, redemption price, percentage	105.438%
Debt Instrument, Redemption, Period Four [Member] | Alliance Capital Trust II [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Debt instrument, redemption price, percentage	100.00%