Investment securities - Amortized Cost and Estimated Fair Value of Collateralized Mortgage Obligations (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Collateralized mortgage obligations, Amortized cost	$ 1,414,691	$ 1,750,922
Estimated fair value	$ 1,202,243	$ 1,417,822

Collateralized mortgage obligations estimated fair value.

No definition available.

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