Derivatives and Hedging Activities - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2015 USD ($) Counterparty CreditRatings	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)
Credit Derivatives [Line Items]
Derivative assets after effects of bilateral collateral and master netting agreements	$ 157,000,000
Derivative liabilities after effects of bilateral collateral and master netting agreements	34,000,000
Derivative assets not designated as hedging instruments after effects of bilateral collateral and master netting agreements, and a reserve for potential future losses	636,000,000
Derivative liabilities not designated as hedging instruments after effects of bilateral collateral and master netting agreements, and a reserve for potential future losses	710,000,000
Reclassify of after-tax net losses on derivative instruments from AOCI	$ 37,000,000
Length of time hedge in cash flow hedge	12 months
Reclassification of net losses related to terminated cash flow hedges from AOCI to income	$ 1,000,000
Maximum length of time over which forecasted transactions are hedged, years	13 years
Other Comprehensive income unrealized gain on derivatives arising during period net of tax	$ 35,000,000
Cash collateral netted against derivative assets	394,000,000	$ 518,000,000	$ 230,000,000
Collateral netted against derivative liabilities	14,000,000	26,000,000	10,000,000
Cash collateral not netted transferred with clearing organization	112,000,000	56,000,000	51,000,000
Cash collateral not netted transferred from clearing organization	105,000,000	0	0
Gross exposure on derivatives, after taking into account the effects of bilateral collateral and master netting agreements	952,000,000
Net exposure on derivatives, after taking into account, the effects of bilateral collateral and master netting agreements	303,000,000
Over-collateralization on derivatives to broker-dealers and banks, after the application of master netting agreements and collateral	172,000,000
Additional collateral held in the form of securities	131,000,000
Default reserve associated with uncollateralized contracts	8,000,000
Gross exposure on derivatives after taking into account effects of master netting agreements	525,000,000
Net exposure on derivatives with clients after application of master netting agreements collateral and related reserve	490,000,000
Letter of credit	$ 30,000,000
Number of counter parties guaranteed | Counterparty	1
Key Corp [Member]
Credit Derivatives [Line Items]
Net liability position totaled	$ 8,000,000
Cash and securities collateral posted	8,000,000
Key Corp [Member] | Maximum [Member]
Credit Derivatives [Line Items]
Additional collateral amount	0	$ 1,000,000	$ 3,000,000
Key Corp [Member] | Maximum [Member] | Unsecured Debt [Member]
Credit Derivatives [Line Items]
Payments to terminate contracts	1,000,000
KeyBank (consolidated) [Member]
Credit Derivatives [Line Items]
Net liability position totaled	249,000,000
Derivative assets included in net liability position	216,000,000
Derivative liabilities included in net liability position	465,000,000
Cash and securities collateral posted	$ 232,000,000
Number of ratings above noninvestment | CreditRatings	4
KeyBank (consolidated) [Member] | Maximum [Member] | Unsecured Debt [Member]
Credit Derivatives [Line Items]
Payments to terminate contracts	$ 7,000,000