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Significant accounting policies (Policies) | 9 Months Ended
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Sep. 30, 2025
Accounting Policies [Abstract] |
Basis of Accounting | The consolidated interim financial statements of the Company were compiled in accordance with GAAP using the accounting policies set forth in note 1 of Notes to Financial Statements included in M&T's 2024 Annual Report. The financial statements contain all adjustments which are, in the opinion of management, necessary for a fair statement of the Company's financial position, results of operations and cash flows for the interim periods presented. The following table provides a description of accounting standards applicable to M&T but not yet adopted at September 30, 2025. Recent accounting developments
| Standard |  |  | Description |  |  | Required date of adoption |  |  | Effect on consolidated financial statements |
| Standards not yet adopted as of September 30, 2025 |
| Income Taxes - Improvements to income tax disclosures |  |  | The standard requires enhanced disclosures in the notes to financial statements including income taxes paid by jurisdiction (federal, state, foreign) and a tabular rate reconciliation between the reported amount of income tax expense (or benefit) and the amount of statutory federal income tax at current rates. |  |  | December 31, 2025 |  |  | The Company intends to make the required disclosures of the amended guidance in its consolidated financial statements for the year ended December 31, 2025. |
| Income Statement - Expense disaggregation disclosures |  |  | The standard requires disclosure in the notes to financial statements of specified information about certain cost and expense captions on the income statement. |  |  | January 1, 2027 |  |  | The Company does not expect the guidance will have a material impact on its consolidated financial statements. |
| Targeted improvements to the accounting for internal-use software |  |  | The standard eliminates the concept of a software development project stage such that the guidance is agnostic to different software development methods and introduces a new threshold for cost capitalization. The standard also provides factors to consider when determining whether significant development uncertainty exists. |  |  | January 1, 2028 |  |  | The Company is evaluating the impact of the standard but does not expect the guidance will have a material impact on its consolidated financial statements. |

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- Definition Disclosure of accounting policy for basis of accounting, or basis of presentation, used to prepare the financial statements (for example, US Generally Accepted Accounting Principles, Other Comprehensive Basis of Accounting, IFRS). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 235 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147483426/235-10-50-4 + Details
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