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
Sep. 30, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]    
Total AFS securities $ 1,911,242 $ 971,339
Gross Unrealized Gains 4,560 10,569
Gross Unrealized (Losses) (32,661) (7,686)
Estimated Fair Value 1,883,141 974,222
Obligations of States and Political Subdivisions    
Debt Securities, Available-for-sale [Line Items]    
Total AFS securities 566,268 295,546
Gross Unrealized Gains 3,380 6,842
Gross Unrealized (Losses) (6,175) (564)
Estimated Fair Value 563,473 301,824
Corporate and Other Bonds    
Debt Securities, Available-for-sale [Line Items]    
Total AFS securities 163,117 113,625
Gross Unrealized Gains 382 1,131
Gross Unrealized (Losses) (2,007) (876)
Estimated Fair Value 161,492 113,880
Mortgage-Backed Securities    
Debt Securities, Available-for-sale [Line Items]    
Total AFS securities 1,170,410 552,431
Gross Unrealized Gains 798 2,596
Gross Unrealized (Losses) (24,295) (6,169)
Estimated Fair Value 1,146,913 548,858
Other Securities    
Debt Securities, Available-for-sale [Line Items]    
Total AFS securities 11,447 9,737
Gross Unrealized Gains 0 0
Gross Unrealized (Losses) (184) (77)
Estimated Fair Value $ 11,263 $ 9,660