Annual report pursuant to Section 13 and 15(d)

SECURITIES (Amortized Cost, Gross Unrealized Gains and Losses, and Estimated Fair Values of Investment Securities) (Details)

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SECURITIES (Amortized Cost, Gross Unrealized Gains and Losses, and Estimated Fair Values of Investment Securities) (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract]    
Total AFS securities, Amortized Cost $ 1,893,427 $ 1,780,952
Gross Unrealized Gains 53,918 9,414
Gross Unrealized (Losses) (1,900) (15,545)
Total AFS securities, Estimated Fair Value 1,945,445 1,774,821
U.S. government and agency securities    
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract]    
Total AFS securities, Amortized Cost 4,487  
Gross Unrealized Gains 11  
Gross Unrealized (Losses) 0  
Total AFS securities, Estimated Fair Value 4,498  
Obligations of states and political subdivisions    
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract]    
Total AFS securities, Amortized Cost 417,397 466,588
Gross Unrealized Gains 25,624 3,844
Gross Unrealized (Losses) (29) (1,941)
Total AFS securities, Estimated Fair Value 442,992 468,491
Corporate and other bonds    
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract]    
Total AFS securities, Amortized Cost 259,213 167,561
Gross Unrealized Gains 4,403 1,118
Gross Unrealized (Losses) (546) (983)
Total AFS securities, Estimated Fair Value 263,070 167,696
Mortgage-backed securities    
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract]    
Total AFS securities, Amortized Cost 1,209,251 1,138,034
Gross Unrealized Gains 23,880 4,452
Gross Unrealized (Losses) (1,325) (12,621)
Total AFS securities, Estimated Fair Value 1,231,806 1,129,865
Other securities    
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract]    
Total AFS securities, Amortized Cost 3,079 8,769
Gross Unrealized Gains 0 0
Gross Unrealized (Losses) 0 0
Total AFS securities, Estimated Fair Value $ 3,079 $ 8,769