SECURITIES (Amortized Cost, Gross Unrealized Gains and Losses, and Estimated Fair Values of Investment Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Total AFS securities	$ 1,859,349	$ 1,780,952
Gross Unrealized Gains	60,866	9,414
Gross Unrealized (Losses)	(1,356)	(15,545)
Estimated Fair Value	1,918,859	1,774,821
U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Total AFS securities	4,475
Gross Unrealized Gains	15
Gross Unrealized (Losses)	0
Estimated Fair Value	4,490
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Total AFS securities	411,690	466,588
Gross Unrealized Gains	26,935	3,844
Gross Unrealized (Losses)	(12)	(1,941)
Estimated Fair Value	438,613	468,491
Corporate and other bonds
Debt Securities, Available-for-sale [Line Items]
Total AFS securities	224,854	167,561
Gross Unrealized Gains	5,044	1,118
Gross Unrealized (Losses)	(374)	(983)
Estimated Fair Value	229,524	167,696
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Total AFS securities	1,215,263	1,138,034
Gross Unrealized Gains	28,872	4,452
Gross Unrealized (Losses)	(970)	(12,621)
Estimated Fair Value	1,243,165	1,129,865
Other securities
Debt Securities, Available-for-sale [Line Items]
Total AFS securities	3,067	8,769
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	0	0
Estimated Fair Value	$ 3,067	$ 8,769

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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