FAIR VALUE DISCLOSURES OF FINANCIAL ASSETS AND LIABILITIES - Additional Information (Details) - USD ($)	3 Months Ended	12 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities available for sale	$ 1,523,196,000		$ 1,205,079,000
Equity securities unrealized gain		$ 15,346,000
Minimum discount rate on appraisals of collateral securing loan	10.00%
Maximum discount rate on appraisals of collateral securing loan	20.00%
Loan balances	$ 51,206,000		49,352,000
Related reserve	5,882,000		2,258,000
Commitments of lending operations	0		$ 0
Significant Other Observable Inputs (Level 2) \| Federal Agency MBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities available for sale	1,500,000,000
Accounting Standards Update 2016-01
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities unrealized gain	$ 3,800,000

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

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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-Topic 320
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-Section 50
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Maximum amount of loss due to credit risk that, based on the gross fair value of the financial instrument, the entity would incur if parties to the financial instruments that make up the concentration failed completely to perform according to the terms of the contracts and the collateral or other security, if any, for the amount due proved to be of no value to the entity.

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Amount of unrealized gain on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 321
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-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84241550&loc=SL75117546-209714

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Includes amounts for loans individually evaluated for impairment and purchased credit impaired loans that have become impaired since acquisition date.

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Impaired Loans Related Specific Reserve

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