Securities - Additional Information (Details) $ in Billions	Dec. 31, 2025 USD ($)
Investments, Debt and Equity Securities [Abstract]
Securities pledged to secure securities sold under repurchase agreements	$ 18.7

Debt Securities, Available For Sale And Held To Maturity, Pledged

No definition available.

Name:	key_DebtSecuritiesAvailableForSaleAndHeldToMaturityPledged
Namespace Prefix:	key_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant