Derivatives and Hedging Activities - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2016 USD ($) CreditRatings	Dec. 31, 2015 USD ($) CreditRatings	Sep. 30, 2015 USD ($) CreditRatings
Credit Derivatives [Line Items]
Derivative assets after effects of bilateral collateral and master netting agreements	$ 291,000,000
Derivative liabilities after effects of bilateral collateral and master netting agreements	9,000,000
Derivative assets not designated as hedging instruments after effects of bilateral collateral and master netting agreements, and a reserve for potential future losses	1,000,000,000
Derivative liabilities not designated as hedging instruments after effects of bilateral collateral and master netting agreements, and a reserve for potential future losses	859,000,000
Reclassify of after-tax net losses on derivative instruments from AOCI	$ 26,000,000
Length of time hedge in cash flow hedge	12 months
Reclassification of net losses related to terminated cash flow hedges from AOCI to income	$ 2,000,000
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	$ 35,000,000
Cash collateral netted against derivative assets	222,000,000	$ 377,000,000	$ 394,000,000
Collateral netted against derivative liabilities	139,000,000	5,000,000	14,000,000
Cash collateral not netted transferred with clearing organization	750,000,000	143,000,000	112,000,000
Cash collateral not netted transferred from clearing organization	236,000,000	$ 6,000,000	$ 105,000,000
Gross exposure on derivatives, after taking into account the effects of bilateral collateral and master netting agreements	702,000,000
Net exposure on derivatives, after taking into account, the effects of bilateral collateral and master netting agreements	461,000,000
Over-collateralization on derivatives to broker-dealers and banks, after the application of master netting agreements and collateral	397,000,000
Additional collateral held in the form of securities	64,000,000
Default reserve associated with uncollateralized contracts	11,000,000
Gross exposure on derivatives after taking into account effects of master netting agreements	902,000,000
Net exposure on derivatives with clients after application of master netting agreements collateral and related reserve	843,000,000
Banking Subsidiary [Member]
Credit Derivatives [Line Items]
Net liability position totaled	257,000,000
Derivative assets included in net liability position	228,000,000
Derivative liabilities included in net liability position	485,000,000
Cash and securities collateral posted	$ 261,000,000
Banking Subsidiary [Member] | Unsecured Debt [Member]
Credit Derivatives [Line Items]
Number of ratings above noninvestment | CreditRatings	4	4	4
Banking Subsidiary [Member] | Maximum [Member] | Unsecured Debt [Member]
Credit Derivatives [Line Items]
Payments to terminate contracts (less than for the $1 million)	$ 3,000,000	$ 5,000,000	$ 7,000,000
Key Corp [Member]
Credit Derivatives [Line Items]
Net liability position totaled	12,000,000
Cash and securities collateral posted	12,000,000
Additional collateral amount	0	0	0
Key Corp [Member] | Unsecured Debt [Member]
Credit Derivatives [Line Items]
Payments to terminate contracts (less than for the $1 million)	1,000,000	$ 1,000,000	$ 1,000,000
First Niagara Bank, N.A. [Member]
Credit Derivatives [Line Items]
Net liability position totaled	23,000,000
Derivative assets included in net liability position	1,000,000
Cash and securities collateral posted	$ 23,000,000