Investment Securities - Additional Information (Detail)	3 Months Ended	9 Months Ended
Sep. 30, 2020 USD ($) Security	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) Security	Sep. 30, 2019 USD ($)
Amortized Cost And Fair Value Debt Securities [Abstract]
Gross realized gains(loss) on sale of investment securities	$ 0	$ 0	$ 0	$ 0
Gains (losses) on equity securities	$ 2,773,000	$ 3,737,000	$ (11,040,000)	$ 24,489,000
Number of debt securities with aggregate gross unrealized losses \| Security	290		290
Unrealized losses on individual debt securities			$ (29,000,000)
Cost method equity securities	$ 381,000,000		$ 381,000,000

Amortized cost and fair value debt securities.

No definition available.

Name:	mtb_AmortizedCostAndFairValueDebtSecuritiesAbstract
Namespace Prefix:	mtb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of debt securities with gross losses.

+ References

No definition available.

+ Details

Name:	mtb_NumberOfDebtSecuritiesWithGrossLosses
Namespace Prefix:	mtb_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of unrealized and realized gain (loss) on investment in debt and equity securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(9)(a))
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(7)(c))
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

+ Details

Name:	us-gaap_DebtAndEquitySecuritiesGainLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of realized gain (loss) on investment in debt and equity securities.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtAndEquitySecuritiesRealizedGainLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of unrealized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), investment in debt security measured at amortized cost (held-to-maturity) and investment in debt security measured at fair value with change in fair value recognized in net income (trading).

+ References

No definition available.

+ Details

Name:	us-gaap_DebtSecuritiesUnrealizedGainLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value.

+ References

No definition available.

+ Details

Name:	us-gaap_EquitySecuritiesFvNiCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant