Borrowings - Long-term Borrowings (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Debt Instrument [Line Items]
Long-term borrowings	$ 12,605	$ 8,201
M&T
Debt Instrument [Line Items]
Subordinated notes (fixed rate)	0	76
Long-term borrowings	5,143	3,098
M&T Bank
Debt Instrument [Line Items]
Other Long-Term Debt	10	10
Long-term borrowings	7,462	5,103
Senior Notes \| M&T
Debt Instrument [Line Items]
Senior notes	$ 4,710	2,482
Senior Notes \| M&T \| Minimum
Debt Instrument [Line Items]
Interest rate of debt instrument	4.55%
Senior Notes \| M&T \| Maximum
Debt Instrument [Line Items]
Interest rate of debt instrument	7.41%
Senior Notes \| M&T Bank
Debt Instrument [Line Items]
Senior notes	$ 3,745	3,741
Senior Notes \| M&T Bank \| Minimum
Debt Instrument [Line Items]
Interest rate of debt instrument	2.90%
Senior Notes \| M&T Bank \| Maximum
Debt Instrument [Line Items]
Interest rate of debt instrument	5.40%
Fixed Junior Subordinated Debentures \| M&T
Debt Instrument [Line Items]
Interest rate of debt instrument	10.60%
Junior subordinated debentures	$ 7	55
Variable Rate Junior Subordinated Debentures \| M&T
Debt Instrument [Line Items]
Junior subordinated debentures	$ 426	485
Variable Rate Junior Subordinated Debentures \| M&T \| Minimum
Debt Instrument [Line Items]
Interest rate of debt instrument	5.70%
Variable Rate Junior Subordinated Debentures \| M&T \| Maximum
Debt Instrument [Line Items]
Interest rate of debt instrument	8.20%
Subordinated Debt \| M&T Bank
Debt Instrument [Line Items]
Interest rate of debt instrument	3.40%
Subordinated notes (fixed rate)	$ 474	873
FHLB Advances, Variable \| M&T Bank
Debt Instrument [Line Items]
Interest rate of debt instrument	4.74%
Federal Home Loan Bank advances	$ 2,000	0
FHLB Advances, Fixed \| M&T Bank
Debt Instrument [Line Items]
Federal Home Loan Bank advances	$ 4	5
FHLB Advances, Fixed \| M&T Bank \| Minimum
Debt Instrument [Line Items]
Interest rate of debt instrument	0.00%
FHLB Advances, Fixed \| M&T Bank \| Maximum
Debt Instrument [Line Items]
Interest rate of debt instrument	5.98%
Asset Backed Notes \| M&T Bank
Debt Instrument [Line Items]
Asset-backed notes	$ 1,229	$ 474
Asset Backed Notes \| M&T Bank \| Minimum
Debt Instrument [Line Items]
Interest rate of debt instrument	4.86%
Asset Backed Notes \| M&T Bank \| Maximum
Debt Instrument [Line Items]
Interest rate of debt instrument	5.76%
Other Debt \| M&T Bank
Debt Instrument [Line Items]
Interest rate of debt instrument	4.38%

Contractual interest rate for funds borrowed, under the debt agreement.

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