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Fair Value Measurements - Quantitative Information Related to Significant Unobservable Inputs (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Dec. 31, 2013
Privately Issued [Member]        
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]        
Fair value $ 95us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
$ 103us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
$ 696us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
$ 1,850us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Collateralized Debt Obligations [Member]        
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]        
Fair value 47,278us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CollateralizedDebtObligationsMember
50,316us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CollateralizedDebtObligationsMember
61,768us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CollateralizedDebtObligationsMember
63,083us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CollateralizedDebtObligationsMember
Other Assets and Other Liabilities [Member]        
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]        
Fair value 26,230us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= mtb_OtherAssetsAndOtherLiabilitiesMember
17,347us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= mtb_OtherAssetsAndOtherLiabilitiesMember
12,589us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= mtb_OtherAssetsAndOtherLiabilitiesMember
3,941us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= mtb_OtherAssetsAndOtherLiabilitiesMember
Discounted Cash Flows [Member] | Collateralized Debt Obligations [Member]        
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]        
Fair value 47,278us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CollateralizedDebtObligationsMember
/ us-gaap_ValuationTechniqueAxis
= mtb_DiscountedCashFlowsMember
50,316us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CollateralizedDebtObligationsMember
/ us-gaap_ValuationTechniqueAxis
= mtb_DiscountedCashFlowsMember
   
Loss severity 100.00%us-gaap_FairValueInputsLossSeverity
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CollateralizedDebtObligationsMember
/ us-gaap_ValuationTechniqueAxis
= mtb_DiscountedCashFlowsMember
100.00%us-gaap_FairValueInputsLossSeverity
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CollateralizedDebtObligationsMember
/ us-gaap_ValuationTechniqueAxis
= mtb_DiscountedCashFlowsMember
   
Discounted Cash Flows [Member] | Collateralized Debt Obligations [Member] | Weighted Average [Member]        
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]        
Probability of default 45.00%us-gaap_FairValueInputsProbabilityOfDefault
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CollateralizedDebtObligationsMember
/ us-gaap_RangeAxis
= us-gaap_WeightedAverageMember
/ us-gaap_ValuationTechniqueAxis
= mtb_DiscountedCashFlowsMember
36.00%us-gaap_FairValueInputsProbabilityOfDefault
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CollateralizedDebtObligationsMember
/ us-gaap_RangeAxis
= us-gaap_WeightedAverageMember
/ us-gaap_ValuationTechniqueAxis
= mtb_DiscountedCashFlowsMember
   
Discounted Cash Flows [Member] | Collateralized Debt Obligations [Member] | Minimum [Member]        
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]        
Probability of default 12.00%us-gaap_FairValueInputsProbabilityOfDefault
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CollateralizedDebtObligationsMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= mtb_DiscountedCashFlowsMember
12.00%us-gaap_FairValueInputsProbabilityOfDefault
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CollateralizedDebtObligationsMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= mtb_DiscountedCashFlowsMember
   
Discounted Cash Flows [Member] | Collateralized Debt Obligations [Member] | Maximum [Member]        
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]        
Probability of default 57.00%us-gaap_FairValueInputsProbabilityOfDefault
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CollateralizedDebtObligationsMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ValuationTechniqueAxis
= mtb_DiscountedCashFlowsMember
57.00%us-gaap_FairValueInputsProbabilityOfDefault
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CollateralizedDebtObligationsMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ValuationTechniqueAxis
= mtb_DiscountedCashFlowsMember
   
Discounted Cash Flows [Member] | Other Assets and Other Liabilities [Member]        
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]        
Fair value 26,230us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= mtb_OtherAssetsAndOtherLiabilitiesMember
/ us-gaap_ValuationTechniqueAxis
= mtb_DiscountedCashFlowsMember
17,347us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= mtb_OtherAssetsAndOtherLiabilitiesMember
/ us-gaap_ValuationTechniqueAxis
= mtb_DiscountedCashFlowsMember
   
Discounted Cash Flows [Member] | Other Assets and Other Liabilities [Member] | Weighted Average [Member]        
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]        
Commitment expirations 19.00%mtb_PercentageOfFairValueInputsCommitmentExpirations
/ us-gaap_FairValueByAssetClassAxis
= mtb_OtherAssetsAndOtherLiabilitiesMember
/ us-gaap_RangeAxis
= us-gaap_WeightedAverageMember
/ us-gaap_ValuationTechniqueAxis
= mtb_DiscountedCashFlowsMember
17.00%mtb_PercentageOfFairValueInputsCommitmentExpirations
/ us-gaap_FairValueByAssetClassAxis
= mtb_OtherAssetsAndOtherLiabilitiesMember
/ us-gaap_RangeAxis
= us-gaap_WeightedAverageMember
/ us-gaap_ValuationTechniqueAxis
= mtb_DiscountedCashFlowsMember
   
Discounted Cash Flows [Member] | Other Assets and Other Liabilities [Member] | Minimum [Member]        
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]        
Commitment expirations 0.00%mtb_PercentageOfFairValueInputsCommitmentExpirations
/ us-gaap_FairValueByAssetClassAxis
= mtb_OtherAssetsAndOtherLiabilitiesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= mtb_DiscountedCashFlowsMember
0.00%mtb_PercentageOfFairValueInputsCommitmentExpirations
/ us-gaap_FairValueByAssetClassAxis
= mtb_OtherAssetsAndOtherLiabilitiesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= mtb_DiscountedCashFlowsMember
   
Discounted Cash Flows [Member] | Other Assets and Other Liabilities [Member] | Maximum [Member]        
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]        
Commitment expirations 96.00%mtb_PercentageOfFairValueInputsCommitmentExpirations
/ us-gaap_FairValueByAssetClassAxis
= mtb_OtherAssetsAndOtherLiabilitiesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ValuationTechniqueAxis
= mtb_DiscountedCashFlowsMember
96.00%mtb_PercentageOfFairValueInputsCommitmentExpirations
/ us-gaap_FairValueByAssetClassAxis
= mtb_OtherAssetsAndOtherLiabilitiesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ValuationTechniqueAxis
= mtb_DiscountedCashFlowsMember
   
Two Independent Pricing Quotes [Member] | Privately Issued [Member]        
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]        
Fair value $ 95us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
/ us-gaap_ValuationTechniqueAxis
= mtb_TwoIndependentPricingQuotesMember
$ 103us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
/ us-gaap_ValuationTechniqueAxis
= mtb_TwoIndependentPricingQuotesMember