LOANS AND ALLOWANCE FOR PROBABLE LOAN LOSSES - Allowance for Loan Losses, Loan Portfolio, by Impairment Method (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule of Allowance For Loan Losses, Loan Balance, by Impairment Method [Line Items]
Loans individually evaluated for impairment	$ 13,460	$ 36,247
Loans collectively evaluated for impairment	3,516,937	3,230,150
Purchased credit impaired loans	37,807	46,402
Total ending loan balance	3,568,204	3,312,799
Construction real estate loans
Schedule of Allowance For Loan Losses, Loan Balance, by Impairment Method [Line Items]
Loans individually evaluated for impairment	406	12
Loans collectively evaluated for impairment	644,419	507,564
Purchased credit impaired loans	123	156
Total ending loan balance	644,948	507,732
1-4 family residential real estate loans
Schedule of Allowance For Loan Losses, Loan Balance, by Impairment Method [Line Items]
Loans individually evaluated for impairment	1,202	1,215
Loans collectively evaluated for impairment	778,797	782,614
Purchased credit impaired loans	7,563	10,670
Total ending loan balance	787,562	794,499
Commercial real estate loans
Schedule of Allowance For Loan Losses, Loan Balance, by Impairment Method [Line Items]
Loans individually evaluated for impairment	8,593	33,013
Loans collectively evaluated for impairment	1,212,651	1,128,220
Purchased credit impaired loans	28,964	32,885
Total ending loan balance	1,250,208	1,194,118
Commercial loans
Schedule of Allowance For Loan Losses, Loan Balance, by Impairment Method [Line Items]
Loans individually evaluated for impairment	2,797	1,394
Loans collectively evaluated for impairment	397,689	353,036
Purchased credit impaired loans	1,035	2,219
Total ending loan balance	401,521	356,649
Municipal loans
Schedule of Allowance For Loan Losses, Loan Balance, by Impairment Method [Line Items]
Loans individually evaluated for impairment	352	429
Loans collectively evaluated for impairment	383,608	352,941
Purchased credit impaired loans	0	0
Total ending loan balance	383,960	353,370
Loans to individuals
Schedule of Allowance For Loan Losses, Loan Balance, by Impairment Method [Line Items]
Loans individually evaluated for impairment	110	184
Loans collectively evaluated for impairment	99,773	105,775
Purchased credit impaired loans	122	472
Total ending loan balance	$ 100,005	$ 106,431

[Line Items] for Schedule of Allowance For Loan Losses, Loan Balance, by Impairment Method [Table]

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- Definition

The recorded balance, net of any applicable reductions (for example, unamortized accretable yield and allowance for loan losses). This relates to loans not accounted for debt securities with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=118939070&loc=d3e15032-111544

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 15
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118938799&loc=d3e13503-111538

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- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

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The balance of financing receivables that were individually evaluated for impairment.

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- Definition

Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118952595&loc=d3e4428-111522

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