Fair Value Measurements - Assets and Liabilities Measured at Estimated Fair Value on Recurring Basis (Detail) - Recurring - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024
Fair Value Measurements
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading account	$ 96,000	$ 101,000
Investment securities available for sale:	20,799,000	18,849,000
Equity securities	280,000	235,000
Real estate loans held for sale	371,000	521,000
Other assets	239,000	255,000
Total assets	21,785,000	19,961,000
Other liabilities	632,000	822,000
Total liabilities	632,000	822,000
Fair Value Measurements | U.S. Treasury
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	8,085,000	7,931,000
Fair Value Measurements | Commercial
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	4,340,000	3,702,000
Fair Value Measurements | Residential
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	8,370,000	7,214,000
Fair Value Measurements | Other
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	4,000	2,000
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading account	96,000	101,000
Investment securities available for sale:	0	0
Equity securities	280,000	235,000
Real estate loans held for sale	0	0
Other assets	0	0
Total assets	376,000	336,000
Other liabilities	0	0
Total liabilities	0	0
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
Level 1 | Commercial
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	0	0
Level 1 | Residential
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	0	0
Level 1 | Other
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	0	0
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading account	0	0
Investment securities available for sale:	20,799,000	18,849,000
Equity securities	0	0
Real estate loans held for sale	371,000	521,000
Other assets	236,000	251,000
Total assets	21,406,000	19,621,000
Other liabilities	632,000	790,000
Total liabilities	632,000	790,000
Level 2 | U.S. Treasury
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	8,085,000	7,931,000
Level 2 | Commercial
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	4,340,000	3,702,000
Level 2 | Residential
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	8,370,000	7,214,000
Level 2 | Other
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	4,000	2,000
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	3,000	4,000
Total assets	3,000	4,000
Other liabilities	0	32,000
Total liabilities	0	32,000
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
Level 3 | Commercial
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	0	0
Level 3 | Residential
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	0	0
Level 3 | Other
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities available for sale:	$ 0	$ 0