Trust Preferred Securities Issued by Unconsolidated Subsidiaries - Summary of Trust Preferred Securities, Common Stock and Related Debentures (Parenthetical) (Detail) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust preferred securities adjustments related to fair value hedges	$ 72	$ 57	$ 68
Redemption price as the sum of the present values of principal and interest payments, if redeemed before maturity	The greater of: (i) the principal amount, plus any accrued but unpaid interest, or (ii) the sum of the present values of principal and interest payments discounted at the Treasury Rate (as defined in the applicable indenture), plus 20 basis points for KeyCorp Capital II or 25 basis points for KeyCorp Capital III or 50 basis points in the case of redemption upon either a tax or a capital treatment event for either KeyCorp Capital II or KeyCorp Capital III, plus any accrued but unpaid interest.
Debentures adjustments related to financial instrument hedging	$ 72	$ 57	$ 68
KeyCorp Capital I [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Floating interest rate	KeyCorp Capital II and KeyCorp Capital III are fixed. KeyCorp Capital I has a floating interest rate, equal to three-month LIBOR plus 74 basis points, that reprices quarterly.
Floating interest rate equal to three-month LIBOR plus percentage	0.74%