Loans and allowance for loan losses - Loan and Leases Considered Nonaccrual and Interest Income Recognized on Loans (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2026	Mar. 31, 2025	Jan. 01, 2026	Dec. 31, 2025	Jan. 01, 2025
Financing Receivable, Impaired [Line Items]
Amortized Cost with Allowance	$ 845	$ 1,139
Amortized Cost without Allowance	395	401
Total	1,240	1,540	$ 1,252	$ 1,252	$ 1,690
Amortized Cost	1,240	1,540	1,252	1,252	1,690
Interest Income Recognized	12	18
Commercial and industrial
Financing Receivable, Impaired [Line Items]
Amortized Cost with Allowance	466	522
Amortized Cost without Allowance	69	140
Total	535	662	527	527	696
Amortized Cost	535	662	527	527	696
Interest Income Recognized	7	6
Commercial Real Estate \| Commercial
Financing Receivable, Impaired [Line Items]
Amortized Cost with Allowance	201	341
Amortized Cost without Allowance	93	53
Total	294	394	320	320	468
Amortized Cost	294	394	320	320	468
Interest Income Recognized	1	7
Commercial Real Estate \| Residential builder and developer
Financing Receivable, Impaired [Line Items]
Amortized Cost with Allowance	0	1
Amortized Cost without Allowance	0	0
Total	0	1	0	0	2
Amortized Cost	0	1	0	0	2
Interest Income Recognized	0	0
Commercial Real Estate \| Other commercial construction
Financing Receivable, Impaired [Line Items]
Amortized Cost with Allowance	2	23
Amortized Cost without Allowance	8	5
Total	10	28	13	13	66
Amortized Cost	10	28	13	13	66
Interest Income Recognized	0	0
Residential Real Estate \| Residential
Financing Receivable, Impaired [Line Items]
Amortized Cost with Allowance	107	135
Amortized Cost without Allowance	165	149
Total	272	284	264	264	279
Amortized Cost	272	284	264	264	279
Interest Income Recognized	2	3
Consumer \| Home equity lines and loans (e)
Financing Receivable, Impaired [Line Items]
Amortized Cost with Allowance	41	36
Amortized Cost without Allowance	43	42
Total	84	78	82	82	81
Amortized Cost	84	78	82	82	81
Interest Income Recognized	2	2
Consumer \| Recreational finance
Financing Receivable, Impaired [Line Items]
Amortized Cost with Allowance	17	17
Amortized Cost without Allowance	15	9
Total	32	26	30	30	31
Amortized Cost	32	26	30	30	31
Interest Income Recognized	0	0
Consumer \| Automobile
Financing Receivable, Impaired [Line Items]
Amortized Cost with Allowance	7	8
Amortized Cost without Allowance	2	3
Total	9	11	11	11	12
Amortized Cost	9	11	11	11	12
Interest Income Recognized	0	0
Consumer \| Other
Financing Receivable, Impaired [Line Items]
Amortized Cost with Allowance	4	56
Amortized Cost without Allowance	0	0
Total	4	56	5	5	55
Amortized Cost	4	56	$ 5	$ 5	$ 55
Interest Income Recognized	$ 0	$ 0

Financing Receivable, Excluding Accrued Interest, Nonaccrual, With Allowance

No definition available.

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

- Definition

Impaired financing receivable nonaccrual interest income.

+ References

No definition available.

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable on nonaccrual status. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 16
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-16

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable on nonaccrual status with no allowance for credit loss. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 16
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-16

+ Details

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

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=mtb_CommercialRealEstateExcludingResidentialBuilderAndDeveloperAndOtherConstructionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=mtb_ResidentialBuilderAndDeveloperMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=mtb_OtherCommercialConstructionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=mtb_ResidentialRealEstateExcludingAltAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=mtb_HomeEquityLinesAndLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=mtb_RecreationalFinanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_AutomobileLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerOtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: