Quarterly report pursuant to Section 13 or 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
Mar. 31, 2020
Dec. 31, 2019
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract]    
Total AFS securities $ 1,900,313 $ 1,893,427
Gross Unrealized Gains 81,556 53,918
Gross Unrealized (Losses) (8,966) (1,900)
Total AFS securities, Estimated Fair Value 1,972,903 1,945,445
U.S. government and agency securities    
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract]    
Total AFS securities 17,631 21,149
Gross Unrealized Gains 365 209
Gross Unrealized (Losses) (34) (38)
Total AFS securities, Estimated Fair Value 17,962 21,320
Obligations of states and political subdivisions    
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract]    
Total AFS securities 515,914 421,344
Gross Unrealized Gains 31,367 25,776
Gross Unrealized (Losses) (555) (29)
Total AFS securities, Estimated Fair Value 546,726 447,091
Corporate and other bonds    
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract]    
Total AFS securities 122,232 134,342
Gross Unrealized Gains 2,185 1,991
Gross Unrealized (Losses) (3,449) (374)
Total AFS securities, Estimated Fair Value 120,968 135,959
Total mortgage-back securities    
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract]    
Total AFS securities 1,241,446 1,313,513
Gross Unrealized Gains 47,639 25,942
Gross Unrealized (Losses) (4,928) (1,459)
Total AFS securities, Estimated Fair Value 1,284,157 1,337,996
Commercial    
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract]    
Total AFS securities 353,684 416,904
Gross Unrealized Gains 13,819 8,786
Gross Unrealized (Losses) (56) (643)
Total AFS securities, Estimated Fair Value 367,447 425,047
Residential    
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract]    
Total AFS securities 887,762 896,609
Gross Unrealized Gains 33,820 17,156
Gross Unrealized (Losses) (4,872) (816)
Total AFS securities, Estimated Fair Value 916,710 912,949
Other securities    
Debt Securities, Available-for-sale Securities, Amortized Cost Basis [Abstract]    
Total AFS securities 3,090 3,079
Gross Unrealized Gains 0 0
Gross Unrealized (Losses) 0 0
Total AFS securities, Estimated Fair Value $ 3,090 $ 3,079