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Securities - Schedule of Amortized Cost, Fair Value and Gross Unrealized Gains and Losses for Securities Held to Maturity (Detail) - USD ($)
$ in Thousands
Mar. 31, 2026
Dec. 31, 2025
Schedule Of Held To Maturity Securities [Line Items]    
Amortized Cost $ 5,703,238 $ 5,724,227
Gross Unrealized Gains 13,006 27,075
Gross Unrealized Losses (543,680) (500,837)
Held to Maturity, Fair value 5,172,564 5,250,465
Allowance for Credit Losses (3,357) (1,684)
Net Carrying Amount 5,699,881 5,722,543
U.S. Treasury [Member]    
Schedule Of Held To Maturity Securities [Line Items]    
Amortized Cost 38,256 38,253
Gross Unrealized Gains   27
Gross Unrealized Losses (298) (37)
Held to Maturity, Fair value 37,958 38,243
Net Carrying Amount 38,256 38,253
Mortgage-backed [Member]    
Schedule Of Held To Maturity Securities [Line Items]    
Amortized Cost 2,463,304 2,513,667
Gross Unrealized Gains 87 335
Gross Unrealized Losses (307,694) (305,040)
Held to Maturity, Fair value 2,155,697 2,208,962
Net Carrying Amount 2,463,304 2,513,667
State and political subdivisions [Member]    
Schedule Of Held To Maturity Securities [Line Items]    
Amortized Cost 3,201,678 3,172,307
Gross Unrealized Gains 12,919 26,713
Gross Unrealized Losses (235,688) (195,760)
Held to Maturity, Fair value 2,978,909 3,003,260
Allowance for Credit Losses (3,357) (1,684)
Net Carrying Amount $ 3,198,321 $ 3,170,623