Variable interest entities and asset securitizations - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Variable Interest Entity [Line Items]
Loss on securitization of assets	$ 0
Loans and leases	135,581,000,000	$ 134,068,000,000
Long-term borrowings	12,605,000,000	8,201,000,000
Other assets for its "investment" in the common securities recognized by the company of various trusts	17,000,000	22,000,000
Total assets of partnerships in which the company invested	$ 208,105,000,000	208,264,000,000
Investment Program, Proportional Amortization Method, Elected, Income Tax Credit and Other Income Tax Benefit, before Amortization, Statement of Income or Comprehensive Income [Extensible Enumeration]	Interest Expense, Operating
Investment Program, Proportional Amortization Method, Elected, Income Tax Credit and Other Income Tax Benefit, before Amortization, Statement of Cash Flows [Extensible Enumeration]	Interest Expense, Operating
Investments amortized to income tax expense	$ 181,000,000	167,000,000	$ 140,000,000
Variable Interest Entity Primary Beneficiary
Variable Interest Entity [Line Items]
Total assets of partnerships in which the company invested	10,500,000,000	9,800,000,000
Federal tax credits and other federal tax benefits recognized	38,000,000	26,000,000	22,000,000
Investments amortized to income tax expense	181,000,000	167,000,000	140,000,000
Maximum exposure to loss of investments in real estate partnerships	2,000,000,000.0
Variable Interest Entity Primary Beneficiary | Third Party Investors
Variable Interest Entity [Line Items]
Long-term borrowings		1,200,000,000
Variable Interest Entity Primary Beneficiary | M&T Bank
Variable Interest Entity [Line Items]
Loans and leases	1,500,000,000
Variable Interest Entity Primary Beneficiary | Renewable Energy
Variable Interest Entity [Line Items]
Federal tax credits and other federal tax benefits recognized	$ 34,000,000	$ 33,000,000	$ 17,000,000