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Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Debt securities available for sale:    
Due in one year or less $ 9,059us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost  
Due after one year through five years 163,114us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost  
Due after five years through ten years 3,272us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost  
Due after ten years 162,001us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost  
Total available for sale (amortized cost) 337,446us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasis  
Mortgage-backed securities available for sale 10,008,287us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost  
Total 10,345,733us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis  
Debt securities held to maturity:    
Due in one year or less 27,663us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount  
Due after one year through five years 87,320us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount  
Due after five years through ten years 33,715us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount  
Due after ten years 7,185us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount  
Total available for held to maturity (amortized cost) 155,883mtb_HeldToMaturitySecuritiesDebtMaturitiesWithSingleMaturityDateNetCarryingAmount  
Mortgage-backed securities held to maturity 3,204,929us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithoutSingleMaturityDateNetCarryingAmount  
Amortized cost for held to maturity 3,360,812us-gaap_HeldToMaturitySecurities 3,507,868us-gaap_HeldToMaturitySecurities
Debt securities available for sale:    
Due in one year or less 9,117us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue  
Due after one year through five years 165,027us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue  
Due after five years through ten years 3,314us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue  
Due after ten years 162,177us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue  
Total available for sale (fair value) 339,635us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDate  
Mortgage-backed securities available for sale 10,265,316us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue  
Total 10,604,951us-gaap_AvailableForSaleSecuritiesDebtSecurities  
Debt securities held to maturity:    
Due in one year or less 27,865us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue  
Due after one year through five years 88,357us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue  
Due after five years through ten years 34,304us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue  
Due after ten years 7,185us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue  
Total available for held to maturity (fair value) 157,711us-gaap_HeldToMaturitySecuritiesDebtMaturitiesSingleMaturityDateFairValue  
Mortgage-backed securities held to maturity 3,254,123us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue  
Total $ 3,411,834us-gaap_HeldToMaturitySecuritiesFairValue $ 3,538,282us-gaap_HeldToMaturitySecuritiesFairValue