INVESTMENT SECURITIES - Amortized Cost and Estimated Fair Value of Investment Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,184,294	$ 813,293
Gross Unrealized Gains	1,668	4,472
Gross Unrealized Losses	(23,392)	(6,892)
Estimated Fair Value	1,162,570	810,873
State, county and municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	151,934	135,968
Gross Unrealized Gains	696	1,989
Gross Unrealized Losses	(881)	(163)
Estimated Fair Value	151,749	137,794
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	67,175	46,659
Gross Unrealized Gains	502	721
Gross Unrealized Losses	(559)	(237)
Estimated Fair Value	67,118	47,143
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	965,185	630,666
Gross Unrealized Gains	470	1,762
Gross Unrealized Losses	(21,952)	(6,492)
Estimated Fair Value	$ 943,703	$ 625,936

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)
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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)
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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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