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Investment Securities - Amortized Cost and Estimated Fair Value of Collateralized Mortgage Obligations (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Amortized Cost and Fair Value Debt Securities [Abstract]    
Collateralized mortgage obligations, Amortized cost $ 209,107mtb_CollateralizedMortgageObligationsAmortizedCost $ 231,040mtb_CollateralizedMortgageObligationsAmortizedCost
Collateralized mortgage obligations, Estimated fair value $ 165,860mtb_CollateralizedMortgageObligationsEstimatedFairValue $ 171,100mtb_CollateralizedMortgageObligationsEstimatedFairValue