Trust Preferred Securities Issued by Unconsolidated Subsidiaries - Summary of Trust Preferred Securities, Common Stock and Related Debentures (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust preferred securities adjustments related to fair value hedges	$ 51	$ 77	$ 83
Redemption price as the sum of the present values of principal and interest payments, if redeemed before maturity	The greater of: (a) the principal amount, plus any accrued but unpaid interest or (b) 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 (25 basis points or 50 basis points in the case of redemption upon either a tax event or a capital treatment event for KeyCorp Capital III), plus any accrued but unpaid interest
Debentures adjustments related to financial instrument hedging	$ 51	$ 77	$ 83
KeyCorp Capital I [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Floating interest rate	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%