INVESTMENT SECURITIES - Amortized Cost and Estimated Fair Value of Investment Securities Available for Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,236,152	$ 1,198,976
Gross Unrealized Gains	7,591	6,257
Gross Unrealized Losses	(9,308)	(12,810)
Estimated Fair Value	1,234,435	1,192,423
State, county and municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	106,468	149,670
Gross Unrealized Gains	1,312	1,367
Gross Unrealized Losses	(40)	(304)
Estimated Fair Value	107,740	150,733
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	56,901	67,123
Gross Unrealized Gains	412	718
Gross Unrealized Losses	(161)	(527)
Estimated Fair Value	57,152	67,314
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,072,783	982,183
Gross Unrealized Gains	5,867	4,172
Gross Unrealized Losses	(9,107)	(11,979)
Estimated Fair Value	$ 1,069,543	$ 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).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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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).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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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
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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