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Investment Securities - Additional Information (Detail) (USD $)
3 Months Ended
Mar. 31, 2015
Investment
Mar. 31, 2014
Dec. 31, 2014
Amortized Cost and Fair Value Debt Securities [Abstract]      
Gross realized gain or loss on sale of Investment Securities $ 0mtb_GrossRealizedGainLossOnSaleOfInvestmentSecurities $ 0mtb_GrossRealizedGainLossOnSaleOfInvestmentSecurities  
Number of investment securities with aggregate gross unrealized losses 294mtb_NumberOfInvestmentSecuritiesWithGrossLosses    
Unrealized losses on individual investment securities 70,985,000mtb_InvestmentSecuritiesGrossUnrealizedLosses   73,008,000mtb_InvestmentSecuritiesGrossUnrealizedLosses
Cost method investment securities $ 329,000,000us-gaap_CostMethodInvestments