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
Jun. 30, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost $ 1,571,735 $ 971,339
Gross Unrealized Gains 7,528 10,569
Gross Unrealized (Losses) (21,215) (7,686)
Estimated Fair Value 1,558,048 974,222
Obligations of States and Political Subdivisions    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 525,328 295,546
Gross Unrealized Gains 5,635 6,842
Gross Unrealized (Losses) (2,379) (564)
Estimated Fair Value 528,584 301,824
Corporate and Other Bonds    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 148,933 113,625
Gross Unrealized Gains 681 1,131
Gross Unrealized (Losses) (1,166) (876)
Estimated Fair Value 148,448 113,880
Mortgage-Backed Securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 885,734 552,431
Gross Unrealized Gains 1,212 2,596
Gross Unrealized (Losses) (17,512) (6,169)
Estimated Fair Value 869,434 548,858
Other Securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 11,740 9,737
Gross Unrealized Gains 0 0
Gross Unrealized (Losses) (158) (77)
Estimated Fair Value $ 11,582 $ 9,660