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Securities - Cumulative Credit Portion of Impairments on Debt Securities (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Investments, Debt and Equity Securities [Abstract]  
Cumulative Impairments on Debt Securities, Beginning Balance $ 4key_CumulativeImpairmentsOnDebtSecurities
Impairment recognized in earnings 0key_ImpairmentsOnDebtSecurities
Cumulative Impairments on Debt Securities, Ending Balance $ 4key_CumulativeImpairmentsOnDebtSecurities