Investment Securities - Amortized Cost and Estimated Fair Value of Collateralized Mortgage Obligations (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Amortized Cost And Fair Value Debt Securities [Abstract]
Collateralized mortgage obligations, Amortized cost	$ 77,964	$ 94,817
Collateralized mortgage obligations, Estimated fair value	$ 71,099	$ 88,410

Fair value of collateralized mortgage obligations included in mortgage-backed securities.

No definition available.

Name:	mtb_CollateralizedMortgageObligationsEstimatedFairValue
Namespace Prefix:	mtb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant