Derivative Financial Instruments - Information about Fair Values of Derivative Instruments in Consolidated Balance Sheet (Parenthetical) (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2020	Dec. 31, 2019
Offsetting [Abstract]
Reduction in estimated fair value of interest rate contracts in asset position	$ (2.9)	$ (43.3)
Reduction in estimated fair value of interest rate contracts in liability position	$ (944.9)	$ (281.3)

Increase (decrease) in estimated fair value of interest rate contracts in liability position.

No definition available.

Name:	mtb_IncreaseDecreaseInEstimatedFairValueOfInterestRateContractsInLiabilityPosition
Namespace Prefix:	mtb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration