v3.26.1
Fair value measurements - Assets and Liabilities Measured at Estimated Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Mar. 31, 2026
Jan. 01, 2026
Dec. 31, 2025
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Residential mortgage loan servicing rights $ 542,000 $ 263,000  
Fair Value Measurements      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Trading account 92,000   $ 97,000
Investment securities available for sale: 25,228,000   23,202,000
Equity securities 281,000   281,000
Real estate loans held for sale 686,000   925,000
Other assets 217,000   245,000
Total assets 27,046,000   24,750,000
Other liabilities 425,000   454,000
Total liabilities 425,000   454,000
Fair Value Measurements | U.S. Treasury      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Investment securities available for sale: 3,240,000   6,343,000
Fair Value Measurements | Commercial      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Investment securities available for sale: 4,782,000   4,816,000
Fair Value Measurements | Residential      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Investment securities available for sale: 17,205,000   12,042,000
Fair Value Measurements | Other      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Investment securities available for sale: 1,000   1,000
Fair Value Measurements | Residential Mortgage Loans      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Residential mortgage loan servicing rights 542,000    
Level 1      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Trading account 12,000   12,000
Investment securities available for sale: 0   0
Equity securities 281,000   281,000
Real estate loans held for sale 0   0
Other assets 0   0
Total assets 293,000   293,000
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 1 | Residential Mortgage Loans      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Residential mortgage loan servicing rights 0    
Level 2      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Trading account 80,000   85,000
Investment securities available for sale: 25,228,000   23,202,000
Equity securities 0   0
Real estate loans held for sale 686,000   925,000
Other assets 215,000   242,000
Total assets 26,209,000   24,454,000
Other liabilities 425,000   454,000
Total liabilities 425,000   454,000
Level 2 | U.S. Treasury      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Investment securities available for sale: 3,240,000   6,343,000
Level 2 | Commercial      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Investment securities available for sale: 4,782,000   4,816,000
Level 2 | Residential      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Investment securities available for sale: 17,205,000   12,042,000
Level 2 | Other      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Investment securities available for sale: 1,000   1,000
Level 2 | Residential Mortgage Loans      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Residential mortgage loan servicing rights 0    
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 2,000   3,000
Total assets 544,000   3,000
Other liabilities 0   0
Total liabilities 0   0
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
Level 3 | Residential Mortgage Loans      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Residential mortgage loan servicing rights $ 542,000