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Commitments and contingencies - Commitments and Contingent Liabilities Outstanding (Detail) - USD ($) $ in Millions | Sep. 30, 2025 | Dec. 31, 2024
--- | --- | ---
Commitments to extend credit |  |
Commercial and industrial | $ 35,781 | $ 31,521
Commercial real estate loans to be sold | 1,074 | 479
Other commercial real estate | 1,956 | 2,697
Residential real estate loans to be sold | 329 | 190
Other residential real estate | 757 | 517
Home equity lines of credit | 7,906 | 7,933
Credit cards | 6,486 | 6,087
Other | 364 | 244
Standby letters of credit | 2,271 | 2,260
Commercial letters of credit | 75 | 58
Financial guarantees and indemnification contracts | 4,552 | 4,335
Commitments to sell real estate loans | $ 1,868 | $ 1,142

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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
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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. + References No definition available. + Details
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Namespace Prefix: | mtb_
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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
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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
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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
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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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- 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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