Capitalized Servicing Assets - Economic Assumptions Used to Determine Fair Value of Capitalized Servicing Rights and Sensitivity of Value to Changes in Assumptions (Detail) - USD ($) $ in Thousands			12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019
Residential Mortgage Loans [Member]
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Weighted-average prepayment speeds			15.84%
Impact on fair value of 10% adverse change	$ (16,555)		$ (16,555)
Impact on fair value of 20% adverse change	$ (31,475)		$ (31,475)
Weighted-average OAS	9.28%		9.28%
Impact on fair value of 10% adverse change	$ (8,070)		$ (8,070)
Impact on fair value of 20% adverse change	$ (15,663)		$ (15,663)
Weighted-average discount rate	10.60%	11.40%
Commercial Mortgage Loans [Member]
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Weighted-average discount rate	18.00%	18.00%	18.00%
Impact on fair value of 10% adverse change	$ (6,948)		$ (6,948)
Impact on fair value of 20% adverse change	$ (13,395)		$ (13,395)

Weighted average option adjusted spread over market implied forward London interbank offered rates used to determine discount rate.

No definition available.

+ Details

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Namespace Prefix:	mtb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Discount rate which is used to value residual cash flows generated by financial assets of a securitization, asset-backed financing arrangement, or similar transfer regardless of when the transfer occurred.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=118948506&loc=d3e107314-111719

+ Details

Name:	us-gaap_AssumptionForFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesDiscountRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Estimated rate of prepayment of principal on financial assets regardless of when the transfer occurred.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(2)
-URI http://asc.fasb.org/extlink&oid=118948506&loc=d3e107314-111719

+ Details

Name:	us-gaap_AssumptionForFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesPrepaymentSpeed
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The hypothetical financial impact of a 10 percent adverse change of the discount rate on the fair value of transferor's interests in transferred financial assets (including any servicing assets or servicing liabilities) as of the balance sheet date.

+ References