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Securities Available for Sale - Amortized Cost and Estimated Fair Value (Details) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Amortized Cost    
Due within one year $ 350  
Due after one year through five years 0  
Due after five years through ten years 19,616  
Due after ten years 97,950  
Amortized Cost 2,228,369 $ 2,239,433
Estimated Fair Value    
Due within one year 351  
Due after one year through five years 0  
Due after five years through ten years 19,478  
Due after ten years 98,791  
Estimated Fair Value 2,233,744 2,285,611
Carrying value of securities pledged to secure public deposits, pledged for various borrowings and for other purposes required or permitted by law 325,000 376,100
Collateralized mortgage obligations    
Amortized Cost    
Amortized Cost 901,735 990,679
Estimated Fair Value    
No single maturity date, fair value 904,659  
Estimated Fair Value 904,659 1,001,317
Mortgage-backed securities: Residential    
Amortized Cost    
Amortized Cost 646,409 672,667
Estimated Fair Value    
No single maturity date, fair value 637,828  
Estimated Fair Value 637,828 681,013
Mortgage-backed securities: Commercial    
Amortized Cost    
Amortized Cost 527,528 482,874
Estimated Fair Value    
No single maturity date, fair value 537,788  
Estimated Fair Value $ 537,788 $ 507,879