Investment Securities - Additional Information (Detail)	3 Months Ended	6 Months Ended
Jun. 30, 2018 USD ($) Security	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) Security	Jun. 30, 2017 USD ($)
Investment Holdings [Line Items]
Gain (loss) on bank investment securities	$ 2,326,000	$ (17,000)	$ (7,105,000)	$ (17,000)
Gross realized gains(loss) on sale of investment securities	$ 0	$ 0	$ 0	$ 0
Number of debt securities with aggregate gross unrealized losses \| Security	1,487		1,487
Unrealized losses on individual debt securities	$ 366,000,000		$ 366,000,000
Cost method equity securities	437,000,000		437,000,000
Retained Earnings [Member] \| Adjustments for New Accounting Pronouncement [Member]
Investment Holdings [Line Items]
Reclassification from accumulated comprehensive income to retained earnings, net of tax	$ 17,000,000		$ 17,000,000

Debt securities gross unrealized losses.

No definition available.

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

- Definition

The aggregate difference between the carrying value and the sale price of investment securities for which the sale price of the investment security is greater than the carrying value of the investment security. This element refers to the gain included in earnings and not to the cash proceeds of the sale.

+ References

No definition available.

+ Details

Name:	mtb_GrossRealizedGainLossOnSaleOfInvestmentSecurities
Namespace Prefix:	mtb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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, after adjustment, of cost-method investment. Adjustments include, but are not limited to, dividends received in excess of earnings after date of investment that are considered a return of investment and other than temporary impairments.

+ References

No definition available.

+ Details

Name:	us-gaap_CostMethodInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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=115205541&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=115205541&loc=SL114868664-224227

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_InvestmentHoldingsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of cumulative effect of the change in accounting principle or new accounting pronouncement on retained earnings or other components of equity or net assets in the statement of financial position as of the beginning of the earliest period presented.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22499-107794

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22583-107794

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22580-107794

+ Details

Name:	us-gaap_NewAccountingPronouncementOrChangeInAccountingPrincipleCumulativeEffectOfChangeOnEquityOrNetAssets1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_RetainedEarningsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_NewAccountingPronouncementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: