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Segment Information - Summary of Segment Information (Detail) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Sales Information [Line Items]      
Average Assets $ 95,892mtb_AverageAsset $ 92,143mtb_AverageAsset $ 86,665mtb_AverageAsset
Business Banking [Member]      
Sales Information [Line Items]      
Average Assets 5,300mtb_AverageAsset
/ us-gaap_StatementBusinessSegmentsAxis
= mtb_BusinessBankingMember
5,281mtb_AverageAsset
/ us-gaap_StatementBusinessSegmentsAxis
= mtb_BusinessBankingMember
5,242mtb_AverageAsset
/ us-gaap_StatementBusinessSegmentsAxis
= mtb_BusinessBankingMember
Commercial Banking [Member]      
Sales Information [Line Items]      
Average Assets 23,683mtb_AverageAsset
/ us-gaap_StatementBusinessSegmentsAxis
= mtb_CommercialBankingMember
22,892mtb_AverageAsset
/ us-gaap_StatementBusinessSegmentsAxis
= mtb_CommercialBankingMember
22,523mtb_AverageAsset
/ us-gaap_StatementBusinessSegmentsAxis
= mtb_CommercialBankingMember
Commercial Real Estate [Member]      
Sales Information [Line Items]      
Average Assets 18,019mtb_AverageAsset
/ us-gaap_StatementBusinessSegmentsAxis
= mtb_CommercialRealEstateSegmentMember
17,113mtb_AverageAsset
/ us-gaap_StatementBusinessSegmentsAxis
= mtb_CommercialRealEstateSegmentMember
16,937mtb_AverageAsset
/ us-gaap_StatementBusinessSegmentsAxis
= mtb_CommercialRealEstateSegmentMember
Discretionary Portfolio [Member]      
Sales Information [Line Items]      
Average Assets 22,714mtb_AverageAsset
/ us-gaap_StatementBusinessSegmentsAxis
= mtb_DiscretionaryPortfolioMember
20,798mtb_AverageAsset
/ us-gaap_StatementBusinessSegmentsAxis
= mtb_DiscretionaryPortfolioMember
18,581mtb_AverageAsset
/ us-gaap_StatementBusinessSegmentsAxis
= mtb_DiscretionaryPortfolioMember
Residential Mortgage Banking [Member]      
Sales Information [Line Items]      
Average Assets 3,512mtb_AverageAsset
/ us-gaap_StatementBusinessSegmentsAxis
= mtb_ResidentialMortgageBankingMember
3,333mtb_AverageAsset
/ us-gaap_StatementBusinessSegmentsAxis
= mtb_ResidentialMortgageBankingMember
3,157mtb_AverageAsset
/ us-gaap_StatementBusinessSegmentsAxis
= mtb_ResidentialMortgageBankingMember
Retail Banking [Member]      
Sales Information [Line Items]      
Average Assets 10,788mtb_AverageAsset
/ us-gaap_StatementBusinessSegmentsAxis
= mtb_RetailBankingMember
10,449mtb_AverageAsset
/ us-gaap_StatementBusinessSegmentsAxis
= mtb_RetailBankingMember
10,155mtb_AverageAsset
/ us-gaap_StatementBusinessSegmentsAxis
= mtb_RetailBankingMember
All Other [Member]      
Sales Information [Line Items]      
Average Assets $ 11,876mtb_AverageAsset
/ us-gaap_ConsolidationItemsAxis
= us-gaap_CorporateNonSegmentMember
$ 12,277mtb_AverageAsset
/ us-gaap_ConsolidationItemsAxis
= us-gaap_CorporateNonSegmentMember
$ 10,070mtb_AverageAsset
/ us-gaap_ConsolidationItemsAxis
= us-gaap_CorporateNonSegmentMember