v3.26.3
Investment Securities - Additional Information (Detail)
$ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2026
USD ($)
Contract
Dec. 31, 2025
USD ($)
Schedule of Investments [Line Items]    
Accrued interest receivable $ 15,608 $ 12,717
Gross unrealized losses on available for sale securities $ 322,001 216,639
Available for sale securities in unrealized loss position | Contract 971  
Available for sale securities in portfolio, number | Contract 1,042  
Capitalization of banks, equal to or greater than, in the single-issue trust preferred portfolio $ 10,000,000  
Equity securities at estimated fair value 25,765 34,760
Carrying value of securities pledged $ 2,130,125 2,102,175
Fitch, AAA Rating [Member]    
Schedule of Investments [Line Items]    
Percentage of asset backed securities with credit rating 13.00%  
Federal Family Education Loan Program [Member] | Fitch, AA+ Rating [Member]    
Schedule of Investments [Line Items]    
Percentage of asset backed securities with credit rating 87.00%  
Minimum [Member] | Federal Family Education Loan Program [Member] | Fitch, AA+ Rating [Member]    
Schedule of Investments [Line Items]    
Percentage of repayment guaranteed by the government 97.00%  
Non Agency Residential Mortgage Backed Securities [Member]    
Schedule of Investments [Line Items]    
Amortized cost of available for sale single issue trust preferred securities $ 32,912  
Available for sale debt securities 0  
Investment Grade [Member]    
Schedule of Investments [Line Items]    
Amortized cost of available for sale single issue trust preferred securities 7,494  
Unrated Bonds Investment Grade [Member]    
Schedule of Investments [Line Items]    
Amortized cost of available for sale single issue trust preferred securities 5,843  
State and Political Subdivisions [Member]    
Schedule of Investments [Line Items]    
Gross unrealized losses on available for sale securities $ 85,935 55,634
Percent of portfolio with credit support 46.00%  
Amortized cost of available for sale single issue trust preferred securities $ 562,533  
Agency Mortgage Backed Securities [Member]    
Schedule of Investments [Line Items]    
Amortized cost of available for sale single issue trust preferred securities 2,608,517  
Commercial Mortgage-Backed Securities [Member]    
Schedule of Investments [Line Items]    
Gross unrealized losses on available for sale securities 23,394 26,052
Amortized cost of available for sale single issue trust preferred securities 324,215  
Residential Mortgage-Backed Securities, Agency [Member]    
Schedule of Investments [Line Items]    
Gross unrealized losses on available for sale securities 194,960 114,317
Amortized cost of available for sale single issue trust preferred securities 2,284,302  
Residential Mortgage-Backed Securities, Non-agency [Member]    
Schedule of Investments [Line Items]    
Gross unrealized losses on available for sale securities 5,236 4,237
Amortized cost of available for sale single issue trust preferred securities $ 32,912  
Residential Mortgage-Backed Securities, Non-agency [Member] | AAA [Member]    
Schedule of Investments [Line Items]    
Percentage of asset backed securities with credit rating 100.00%  
Corporate Bonds [Member]    
Schedule of Investments [Line Items]    
Gross unrealized losses on available for sale securities $ 8,735 11,881
Amortized cost of available for sale single issue trust preferred securities $ 180,268  
Corporate Bonds [Member] | Investment Grade [Member]    
Schedule of Investments [Line Items]    
Percent of corporate securities portfolio 94.00%  
Corporate Bonds [Member] | Below Investment Grade [Member]    
Schedule of Investments [Line Items]    
Percent of corporate securities portfolio 2.00%  
Corporate Bonds [Member] | Unrated Bonds Investment Grade [Member]    
Schedule of Investments [Line Items]    
Percent of corporate securities portfolio 4.00%  
Mortgage Backed Securities [Member]    
Schedule of Investments [Line Items]    
Net unrealized gains $ 223,400  
Asset-backed Securities [Member]    
Schedule of Investments [Line Items]    
Gross unrealized losses on available for sale securities 1,887 $ 2,381
Amortized cost of available for sale single issue trust preferred securities $ 151,445