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Loans and Leases - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Loans And Leases Receivable [Line Items]      
Charges incurred for reimbursement of loan $ 0 $ 0 $ 0
Interest income that would have been recognized on nonaccrual and renegotiated loans if those loans were accruing interest at their originally contracted terms 68,000,000 69,000,000 64,000,000
Interest on nonaccrual and renegotiated loans included in interest income 33,000,000 33,000,000 $ 31,000,000
Contractual principal and interest payments 228,000,000 303,000,000  
Guaranteed amount included in the estimated residual value of leased assets associated with direct financing leases 37,000,000 39,000,000  
Amount of foreclosed residential real estate property held 76,000,000 77,000,000  
Loans secured by residential real estate that were in the process of foreclosure $ 402,000,000 391,000,000  
Percentage loans in the process of foreclosure, serviced by other entities, classified as government guaranteed 36.00%    
Commercial Loans and Leases [Member]      
Loans And Leases Receivable [Line Items]      
Loans pledged to secure outstanding borrowings from the FHLB of New York $ 11,900,000,000    
Residential Mortgage Loans [Member]      
Loans And Leases Receivable [Line Items]      
Loans pledged to secure outstanding borrowings from the FHLB of New York 11,500,000,000    
Home Equity Loans and Lines of Credit [Member]      
Loans And Leases Receivable [Line Items]      
Loans pledged to secure outstanding borrowings from the FHLB of New York 2,300,000,000    
Other Consumer Loans [Member]      
Loans And Leases Receivable [Line Items]      
Loans pledged to secure outstanding borrowings from the FHLB of New York $ 7,500,000,000    
Director and Certain Officers [Member]      
Loans And Leases Receivable [Line Items]      
Exclusion limit of loans from related party disclosure less than $60,000    
Borrowing by directors and certain officers of M&T and its banking subsidiaries and their associates $ 28,000,000 $ 77,000,000  
New borrowings by directors, certain officers and their associates 1,000,000    
Repayments and other reductions of borrowings by directors, certain officers and their associates $ 50,000,000    
Maximum [Member]      
Loans And Leases Receivable [Line Items]      
Purchased impaired loans as a percentage of total assets 1.00% 1.00%  
Commercial Real Estate [Member]      
Loans And Leases Receivable [Line Items]      
Loans serviced for others sold with credit recourse $ 3,900,000,000    
Loans pledged to secure outstanding borrowings from the FHLB of New York 13,300,000,000    
Residential Mortgage Loans [Member]      
Loans And Leases Receivable [Line Items]      
Mortgage loans held for sale 414,000,000 $ 205,000,000  
RealEstateLoanMember      
Loans And Leases Receivable [Line Items]      
Mortgage loans held for sale $ 28,000,000 $ 347,000,000