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Investment Securities - Amortized Cost and Estimated Fair Value of Securities Contractual Maturity (Detail) - USD ($)
$ in Thousands
Mar. 31, 2024
Dec. 31, 2023
Amortized Cost    
Due in one year or less, Amortized Cost $ 15,856  
Due in one year or less, Amortized Cost 0  
Due after one year through five years, Amortized Cost 235,704  
Due after one year through five years, Amortized Cost 28,202  
Due after five years through ten years, Amortized Cost 396,977  
Due after five years through ten years, Amortized Cost 313,321  
Due after ten years, Amortized Cost 890,325  
Due after ten years, Amortized Cost 811,991  
Amortized Cost 3,763,609 $ 3,840,927
Amortized Cost 1,282,591 1,283,987
Estimated Fair Value    
Due in one year or less, Estimated Fair Value 15,686  
Due in one year or less, Estimated Fair Value 0  
Due after one year through five years, Estimated Fair Value 220,573  
Due after one year through five years, Estimated Fair Value 26,487  
Due after five years through ten years, Estimated Fair Value 355,394  
Due after five years through ten years, Estimated Fair Value 283,296  
Due after ten years, Estimated Fair Value 821,615  
Due after ten years, Estimated Fair Value 726,378  
Total, Estimated Fair Value 3,400,884 3,507,841
Estimated Fair Value 1,159,145 1,170,481
U.S. government-sponsored mortgage-backed securities    
Amortized Cost    
Securities not due at a single maturity date, Amortized Cost 1,667,526  
Securities not due at a single maturity date, Amortized Cost 129,077  
Amortized Cost 1,667,526 1,711,668
Amortized Cost 129,077 130,278
Estimated Fair Value    
Securities not due at a single maturity date, Estimated Fair Value 1,452,735  
Securities not due at a single maturity date, Estimated Fair Value 122,984  
Total, Estimated Fair Value 1,452,735 1,520,421
Estimated Fair Value 122,984 126,022
Private mortgage-backed securities    
Amortized Cost    
Securities not due at a single maturity date, Amortized Cost 190,155  
Securities not due at a single maturity date, Amortized Cost 0  
Amortized Cost 190,155 191,522
Estimated Fair Value    
Securities not due at a single maturity date, Estimated Fair Value 173,541  
Securities not due at a single maturity date, Estimated Fair Value 0  
Total, Estimated Fair Value 173,541 175,405
Non-government-sponsored asset backed securities    
Amortized Cost    
Securities not due at a single maturity date, Amortized Cost 367,066  
Securities not due at a single maturity date, Amortized Cost 0  
Amortized Cost 367,066 370,203
Estimated Fair Value    
Securities not due at a single maturity date, Estimated Fair Value 361,340  
Securities not due at a single maturity date, Estimated Fair Value 0  
Total, Estimated Fair Value $ 361,340 $ 363,473