Loans and Leases - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loans And Leases Receivable [Line Items]
Commercial real estate loans held for sale	$ 22,000,000	$ 643,000,000
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	63,872,000	68,371,000	56,784,000
Interest on nonaccrual and renegotiated loans included in interest income	31,425,000	33,941,000	$ 30,735,000
Contractual principal and interest payments	410,000,000	578,000,000
Guaranteed amount included in the estimated residual value of leased assets associated with direct financing leases	37,000,000	47,000,000
Amount of foreclosed residential real estate property held	108,000,000	129,000,000
Loans secured by residential real estate that were in the process of foreclosure	$ 497,000,000	506,000,000
Percentage loans in the process of foreclosure, serviced by other entities, classified as purchased impaired	45.00%
Percentage loans in the process of foreclosure, serviced by other entities, classified as government guaranteed	19.00%
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	$ 93,103,000	$ 63,543,000
New borrowings by directors, certain officers and their associates	88,077,000
Repayments and other reductions of borrowings by directors, certain officers and their associates	$ 58,517,000
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	$ 3,300,000,000
Loans pledged to secure outstanding borrowings from the FHLB of New York	12,700,000,000
Residential Mortgage Loans [Member]
Loans And Leases Receivable [Line Items]
Mortgage loans held for sale	356,000,000	$ 414,000,000
Loans pledged to secure outstanding borrowings from the FHLB of New York	15,300,000,000
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
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	$ 5,200,000,000