INVESTMENT SECURITIES - Amortized Cost and Estimated Fair Value of Investment Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,471,458	$ 1,198,976
Gross Unrealized Gains	22,824	6,257
Gross Unrealized Losses	(3,075)	(12,810)
Estimated Fair Value	1,491,207	1,192,423
U.S. government sponsored agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	22,265
Gross Unrealized Gains	95
Gross Unrealized Losses	0
Estimated Fair Value	22,360
State, county and municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	113,607	149,670
Gross Unrealized Gains	2,742	1,367
Gross Unrealized Losses	0	(304)
Estimated Fair Value	116,349	150,733
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	51,740	67,123
Gross Unrealized Gains	1,211	718
Gross Unrealized Losses	(33)	(527)
Estimated Fair Value	52,918	67,314
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,283,846	982,183
Gross Unrealized Gains	18,776	4,172
Gross Unrealized Losses	(3,042)	(11,979)
Estimated Fair Value	$ 1,299,580	$ 974,376

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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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Amount 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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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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