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Relationship with BLG and Bayview Financial - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Feb. 19, 2025
Schedule of Equity Method Investments [Line Items]
Long-term borrowings	$ 12,605,000,000	$ 8,201,000,000
Loans and leases	135,581,000,000	134,068,000,000
Commercial Portfolio Segment
Schedule of Equity Method Investments [Line Items]
Loans and leases	$ 61,481,000,000	57,010,000,000
Bayview Lending Group
Schedule of Equity Method Investments [Line Items]
Ownership percentage (as a percent)	20.00%
Equity method investments	$ 0
Bayview Lending Group | Other Revenues From Operations
Schedule of Equity Method Investments [Line Items]
Income from BLG	48,000,000	20,000,000	$ 30,000,000
Bayview Lending Group and Bayview Financial
Schedule of Equity Method Investments [Line Items]
Outstanding principal balances of mortgage servicing rights	1,000,000,000.0	1,200,000,000
Revenue from contract with customer	5,000,000	6,000,000	8,000,000
Bayview Financial
Schedule of Equity Method Investments [Line Items]
Outstanding principal balances of residential mortgage loans from Bayview Financial	111,500,000,000	115,300,000,000
Revenues from sub-servicing	123,000,000	125,000,000	$ 154,000,000
Investment securities in held-to-maturity portfolio securitized by Bayview Financial	37,000,000	$ 42,000,000
Bayview Financial | Subsequent Event
Schedule of Equity Method Investments [Line Items]
Outstanding principal balances of residential mortgage loans from Bayview Financial				$ 51,700,000,000
Bayview Financial | Subsequent Event | Commercial Portfolio Segment
Schedule of Equity Method Investments [Line Items]
Loans and leases				$ 333,000,000
Bayview Financial | Syndicated Loan Facility
Schedule of Equity Method Investments [Line Items]
Long-term borrowings	2,700,000,000
Bayview Financial | Syndicated Loan Facility | M&T Bank
Schedule of Equity Method Investments [Line Items]
Long-term borrowings	$ 404,000,000

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