LOANS AND ALLOWANCE FOR PROBABLE LOAN LOSSES - Aging of Past Due Loans By Class of Loan (Details) - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		$ 34,314	$ 19,039
Current	[1]	3,278,485	3,275,317
Total ending loans balance		3,312,799	3,294,356
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		21,966	13,777
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		10,276	3,901
Greater than 90 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		2,072	1,361
Construction Real Estate Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		934	2,900
Current	[1]	506,798	472,967
Total ending loans balance		507,732	475,867
Construction Real Estate Loans | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		627	1,302
Construction Real Estate Loans | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		307	1,530
Construction Real Estate Loans | Greater than 90 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		0	68
1-4 Family Residential Real Estate Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		10,034	10,944
Current	[1]	784,465	794,397
Total ending loans balance		794,499	805,341
1-4 Family Residential Real Estate Loans | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		7,441	8,508
1-4 Family Residential Real Estate Loans | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		1,258	1,574
1-4 Family Residential Real Estate Loans | Greater than 90 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		1,335	862
Commercial Real Estate Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		18,318	1,386
Current	[1]	1,175,800	1,263,773
Total ending loans balance		1,194,118	1,265,159
Commercial Real Estate Loans | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		10,663	1,357
Commercial Real Estate Loans | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		7,655	24
Commercial Real Estate Loans | Greater than 90 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		0	5
Commercial Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		3,242	1,395
Current	[1]	353,407	265,027
Total ending loans balance		356,649	266,422
Commercial Loans | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		1,946	662
Commercial Loans | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		705	400
Commercial Loans | Greater than 90 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		591	333
Municipal Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		0	422
Current	[1]	353,370	345,376
Total ending loans balance		353,370	345,798
Municipal Loans | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		0	422
Municipal Loans | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		0	0
Municipal Loans | Greater than 90 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		0	0
Loans to Individuals
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		1,786	1,992
Current	[1]	104,645	133,777
Total ending loans balance		106,431	135,769
Loans to Individuals | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		1,289	1,526
Loans to Individuals | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		351	373
Loans to Individuals | Greater than 90 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		$ 146	$ 93

[1] Includes PCI loans measured at fair value at acquisition if the timing and amount of cash flows expected to be collected from those sales can be reasonably estimated.