v3.26.3
Investment Securities - Summary of Amortized Cost and Estimated Fair Values of Available for Sale Securities (Detail) - USD ($)
$ in Thousands
Sep. 30, 2026
Dec. 31, 2025
Schedule of Investments [Line Items]    
Amortized Cost $ 3,822,245 $ 3,264,860
Gross Unrealized Gains 251 11,231
Gross Unrealized Losses 322,001 216,639
Allowance For Credit Losses 0 0
Estimated Fair Value 3,500,495 3,059,452
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member]    
Schedule of Investments [Line Items]    
Amortized Cost 273,233 283,058
Gross Unrealized Gains 17 75
Gross Unrealized Losses 1,501 1,476
Allowance For Credit Losses 0 0
Estimated Fair Value 271,749 281,657
State and Political Subdivisions [Member]    
Schedule of Investments [Line Items]    
Amortized Cost 562,533 572,217
Gross Unrealized Gains 6 343
Gross Unrealized Losses 85,935 55,634
Allowance For Credit Losses 0 0
Estimated Fair Value 476,604 516,926
Residential mortgage-backed securities Agency [Member]    
Schedule of Investments [Line Items]    
Amortized Cost 2,284,302 1,467,436
Gross Unrealized Gains 5 5,517
Gross Unrealized Losses 194,960 114,317
Allowance For Credit Losses 0 0
Estimated Fair Value 2,089,347 1,358,636
Residential Mortgage-Backed Securities, Non-agency [Member]    
Schedule of Investments [Line Items]    
Amortized Cost 32,912 42,792
Gross Unrealized Gains 153 330
Gross Unrealized Losses 5,236 4,237
Allowance For Credit Losses 0 0
Estimated Fair Value 27,829 38,885
Commercial Mortgage-Backed Securities Agency [Member]    
Schedule of Investments [Line Items]    
Amortized Cost 324,215 416,177
Gross Unrealized Gains 32 4,948
Gross Unrealized Losses 23,394 26,052
Allowance For Credit Losses 0 0
Estimated Fair Value 300,853 395,073
Asset-backed Securities [Member]    
Schedule of Investments [Line Items]    
Amortized Cost 151,445 225,617
Gross Unrealized Gains 38 18
Gross Unrealized Losses 1,887 2,381
Allowance For Credit Losses 0 0
Estimated Fair Value 149,596 223,254
Single Issue Trust Preferred Securities [Member]    
Schedule of Investments [Line Items]    
Amortized Cost 13,337 13,319
Gross Unrealized Gains 0 0
Gross Unrealized Losses 353 661
Allowance For Credit Losses 0 0
Estimated Fair Value 12,984 12,658
Other Corporate Securities [Member]    
Schedule of Investments [Line Items]    
Amortized Cost 180,268 244,244
Gross Unrealized Gains 0 0
Gross Unrealized Losses 8,735 11,881
Allowance For Credit Losses 0 0
Estimated Fair Value $ 171,533 $ 232,363