Derivatives and Hedging Activities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016 USD ($) rating	Dec. 31, 2015 USD ($) rating
Credit Derivatives [Line Items]
Derivative assets after effects of bilateral collateral and master netting agreements	$ 85,000,000
Derivative liabilities after effects of bilateral collateral and master netting agreements	66,000,000
Derivative assets not designated as hedging instruments after effects of bilateral collateral and master netting agreements, and a reserve for potential future losses	718,000,000
Derivative liabilities not designated as hedging instruments after effects of bilateral collateral and master netting agreements, and a reserve for potential future losses	570,000,000
Reclassify of after-tax net losses on derivative instruments from AOCI	$ 13,000,000
Length of time hedge in cash flow hedge	12 months
Reclassification of net losses related to terminated cash flow hedges from AOCI to income	$ 3,000,000
Maximum length of time over which forecasted transactions are hedged, years	12 years
Other comprehensive income unrealized gain on derivatives arising during period net of tax	$ 40,000,000
Cash collateral netted against derivative assets	155,000,000	$ 377,000,000
Collateral netted against derivative liabilities	108,000,000	5,000,000
Cash collateral not netted transferred with clearing organization	448,000,000	143,000,000
Cash collateral not netted transferred from clearing organization	59,000,000	6,000,000
Gross exposure on derivatives, after taking into account the effects of bilateral collateral and master netting agreements	536,000,000
Net exposure on derivatives, after taking into account, the effects of bilateral collateral and master netting agreements	330,000,000
Over-collateralization on derivatives to broker-dealers and banks, after the application of master netting agreements and collateral	287,000,000
Additional collateral held in the form of securities	43,000,000
Default reserve associated with uncollateralized contracts	6,000,000
Gross exposure on derivatives after taking into account effects of master netting agreements	531,000,000
Net exposure on derivatives with clients after application of master netting agreements collateral and related reserve	473,000,000
KeyBank (consolidated) [Member]
Credit Derivatives [Line Items]
Net liability position totaled	199,000,000
Derivative assets included in net liability position	155,000,000
Derivative liabilities included in net liability position	354,000,000
Cash and securities collateral posted	196,000,000
Additional collateral amount	$ 1,000,000	$ 1,000,000
KeyBank (consolidated) [Member] | Unsecured Debt [Member]
Credit Derivatives [Line Items]
Number of ratings above noninvestment | rating	4	4
KeyBank (consolidated) [Member] | Maximum [Member] | Unsecured Debt [Member]
Credit Derivatives [Line Items]
Payments to terminate contracts	$ 4,000,000	$ 5,000,000
Key Corp [Member]
Credit Derivatives [Line Items]
Net liability position totaled	6,000,000
Cash and securities collateral posted	6,000,000
Additional collateral amount	$ 0	0
Key Corp [Member] | Maximum [Member] | Unsecured Debt [Member]
Credit Derivatives [Line Items]
Payments to terminate contracts		$ 1,000,000