v3.26.1
Fair value measurements - Assets and Liabilities Measured at Estimated Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Jun. 30, 2026
Jan. 01, 2026
Dec. 31, 2025
Mar. 31, 2025
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]        
Residential mortgage loan servicing rights $ 540,000 $ 263,000   $ 542,000
Fair Value Measurements        
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]        
Investment securities available for sale: 25,370,000   $ 23,202,000  
Equity securities 245,000   281,000  
Real estate loans held for sale 515,000   925,000  
Other assets 332,000   342,000  
Total assets 27,002,000   24,750,000  
Other liabilities 503,000   454,000  
Total liabilities 503,000   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: 3,227,000   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: 4,755,000   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: 17,387,000   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 540,000      
Level 1        
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]        
Investment securities available for sale: 0   0  
Equity securities 245,000   281,000  
Real estate loans held for sale 0   0  
Other assets 13,000   12,000  
Total assets 258,000   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]        
Investment securities available for sale: 25,370,000   23,202,000  
Equity securities 0   0  
Real estate loans held for sale 515,000   925,000  
Other assets 315,000   327,000  
Total assets 26,200,000   24,454,000  
Other liabilities 503,000   454,000  
Total liabilities 503,000   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: 3,227,000   6,343,000  
Level 2 | Commercial        
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]        
Investment securities available for sale: 4,755,000   4,816,000  
Level 2 | Residential        
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]        
Investment securities available for sale: 17,387,000   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]        
Investment securities available for sale: 0   0  
Equity securities 0   0  
Real estate loans held for sale 0   0  
Other assets 4,000   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 $ 540,000