Fair value measurements - Assets and Liabilities Measured at Estimated Fair Value on Recurring Basis (Detail) - USD ($)	Sep. 30, 2025	Dec. 31, 2024
Level 3 | Measurement Input, Commitment Expiration
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Held-for-sale measurement input (as a percent)	0.26	0.06
Recurring | Fair Value Measurements
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading account	$ 95,000,000	$ 101,000,000
Investment securities available for sale:	23,163,000,000	18,849,000,000
Equity securities	258,000,000	235,000,000
Real estate loans held for sale	605,000,000	521,000,000
Other assets	253,000,000	255,000,000
Total assets	24,374,000,000	19,961,000,000
Other liabilities	480,000,000	822,000,000
Total liabilities	480,000,000	822,000,000
Recurring | Fair Value Measurements | U.S. Treasury
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	6,620,000,000	7,931,000,000
Recurring | Fair Value Measurements | Commercial
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	4,861,000,000	3,702,000,000
Recurring | Fair Value Measurements | Residential
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	11,680,000,000	7,214,000,000
Recurring | Fair Value Measurements | Other
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	2,000,000	2,000,000
Recurring | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading account	95,000,000	101,000,000
Investment securities available for sale:	0	0
Equity securities	258,000,000	235,000,000
Real estate loans held for sale	0	0
Other assets	0	0
Total assets	353,000,000	336,000,000
Other liabilities	0	0
Total liabilities	0	0
Recurring | Level 1 | U.S. Treasury
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	0	0
Recurring | Level 1 | Commercial
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	0	0
Recurring | Level 1 | Residential
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	0	0
Recurring | Level 1 | Other
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	0	0
Recurring | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading account	0	0
Investment securities available for sale:	23,163,000,000	18,849,000,000
Equity securities	0	0
Real estate loans held for sale	605,000,000	521,000,000
Other assets	248,000,000	251,000,000
Total assets	24,016,000,000	19,621,000,000
Other liabilities	480,000,000	790,000,000
Total liabilities	480,000,000	790,000,000
Recurring | Level 2 | U.S. Treasury
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	6,620,000,000	7,931,000,000
Recurring | Level 2 | Commercial
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	4,861,000,000	3,702,000,000
Recurring | Level 2 | Residential
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	11,680,000,000	7,214,000,000
Recurring | Level 2 | Other
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	2,000,000	2,000,000
Recurring | Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading account	0	0
Investment securities available for sale:	0	0
Equity securities	0	0
Real estate loans held for sale	0	0
Other assets	5,000,000	4,000,000
Total assets	5,000,000	4,000,000
Other liabilities	0	32,000,000
Total liabilities	0	32,000,000
Recurring | Level 3 | U.S. Treasury
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	0	0
Recurring | Level 3 | Commercial
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	0	0
Recurring | Level 3 | Residential
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	0	0
Recurring | Level 3 | Other
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	$ 0	$ 0