Fair value measurements - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Jun. 30, 2025	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Change in fair value of nonrecurring fair value measured loans for charge-offs and impairment reserves	$ 67,000,000	$ 81,000,000		$ 154,000,000	$ 204,000,000
Level 3 \| Residential Mortgage Loans
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Capitalized servicing rights, valuation allowance			$ 0	0	0
Nonrecurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Loans measured at fair value on nonrecurring basis	577,000,000	805,000,000		577,000,000	805,000,000	$ 847,000,000
Nonrecurring \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Loans measured at fair value on nonrecurring basis	283,000,000	159,000,000		283,000,000	159,000,000	187,000,000
Nonrecurring \| Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Loans measured at fair value on nonrecurring basis	$ 294,000,000	$ 646,000,000		$ 294,000,000	$ 646,000,000	$ 660,000,000

M&T BANK CORP 10-Q