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Commitments and contingencies - Commitments and Contingent Liabilities Outstanding (Detail) - USD ($) $ in Millions | Jun. 30, 2026 | Dec. 31, 2025
--- | --- | ---
Commitments to extend credit |  |
Commercial and industrial | $ 37,156 | $ 35,654
Commercial real estate loans to be sold | 485 | 773
Other commercial real estate | 3,292 | 2,331
Residential real estate loans to be sold | 258 | 224
Other residential real estate | 794 | 679
Home equity lines of credit | 7,885 | 7,974
Credit cards | 6,762 | 6,601
Other | 354 | 444
Standby letters of credit | 2,332 | 2,318
Commercial letters of credit | 63 | 72
Financial guarantees and indemnification contracts | 4,883 | 4,751
Commitments to sell real estate loans | $ 1,207 | $ 1,898

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- Definition Commitments regarding commercial letters of credit. + References No definition available. + Details
Name: | mtb_CommitmentsRegardingCommercialLettersOfCredit
Namespace Prefix: | mtb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Commitments regarding financial guarantees and indemnification contracts. + References No definition available. + Details
Name: | mtb_CommitmentsRegardingFinancialGuaranteesAndIndemnificationContracts
Namespace Prefix: | mtb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Commitments to extend credit. + References No definition available. + Details
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Namespace Prefix: | mtb_
Data Type: | xbrli:stringItemType
Balance Type: | na
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- Definition Commitments To Extend Credit For Commercial And Industrial + References No definition available. + Details
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Namespace Prefix: | mtb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Commitments to extend credit for commercial and other. + References No definition available. + Details
Name: | mtb_CommitmentsToExtendCreditForCommercialAndOther
Namespace Prefix: | mtb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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- Definition Commitments to extend credit for commercial real estate loans to be sold. + References No definition available. + Details
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Namespace Prefix: | mtb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Commitments To Extend Credit For Credit Cards + References No definition available. + Details
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Namespace Prefix: | mtb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Commitments to extend credit for home equity lines of credit. + References No definition available. + Details
Name: | mtb_CommitmentsToExtendCreditForHomeEquityLinesOfCredit
Namespace Prefix: | mtb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Commitments to extend credit for other commercial real estate. + References No definition available. + Details
Name: | mtb_CommitmentsToExtendCreditForOtherCommercialRealEstate
Namespace Prefix: | mtb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Commitments to extend credit for other residential real estate. + References No definition available. + Details
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Namespace Prefix: | mtb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Commitments to extend credit for residential real estate loans to be sold. + References No definition available. + Details
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Namespace Prefix: | mtb_
Data Type: | xbrli:monetaryItemType
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Period Type: | instant

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- Definition Commitments to sell real estate loans notional amount. + References No definition available. + Details
Name: | mtb_CommitmentsToSellRealEstateLoansNotionalAmount
Namespace Prefix: | mtb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 55 -Paragraph 15 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477548/944-605-55-15 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477548/944-605-55-12 + Details
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