LOANS AND ALLOWANCE FOR PROBABLE LOAN LOSSES - Loans by Credit Quality Indicator (Details) - USD ($) $ in Thousands		Dec. 31, 2018		Dec. 31, 2017
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance		$ 3,312,799		$ 3,294,356
PCI loans		46,402	[1],[2]	45,233
Pass
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance		3,171,320		3,131,151
Pass Watch
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	12,203		39,808
PCI loans		22		362
Special Mention
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	29,783		31,223
PCI loans		859		6,000
Substandard
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	96,950		90,629
PCI loans		3,900		10,500
Doubtful
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	2,543		1,545
PCI loans		1,200		925
Construction Real Estate Loans
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance		507,732		475,867
PCI loans		156		276
Construction Real Estate Loans | Pass
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance		507,529		471,446
Construction Real Estate Loans | Pass Watch
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	163		3,329
Construction Real Estate Loans | Special Mention
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	0		77
Construction Real Estate Loans | Substandard
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	28		982
Construction Real Estate Loans | Doubtful
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	12		33
1-4 Family Residential Real Estate Loans
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance		794,499		805,341
PCI loans		10,670		6,649
1-4 Family Residential Real Estate Loans | Pass
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance		787,516		796,639
1-4 Family Residential Real Estate Loans | Pass Watch
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	37		559
1-4 Family Residential Real Estate Loans | Special Mention
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	100		857
1-4 Family Residential Real Estate Loans | Substandard
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	5,489		6,610
1-4 Family Residential Real Estate Loans | Doubtful
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	1,357		676
Commercial Real Estate Loans
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance		1,194,118		1,265,159
PCI loans		32,885		31,937
Commercial Real Estate Loans | Pass
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance		1,067,874		1,136,576
Commercial Real Estate Loans | Pass Watch
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	11,479		26,275
Commercial Real Estate Loans | Special Mention
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	26,490		25,301
Commercial Real Estate Loans | Substandard
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	87,767		76,625
Commercial Real Estate Loans | Doubtful
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	508		382
Commercial Loans
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance		356,649		266,422
PCI loans		2,219		5,248
Commercial Loans | Pass
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance		349,495		247,430
Commercial Loans | Pass Watch
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	520		9,625
Commercial Loans | Special Mention
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	3,189		3,956
Commercial Loans | Substandard
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	2,988		5,203
Commercial Loans | Doubtful
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	457		208
Municipal Loans
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance		353,370		345,798
PCI loans		0		0
Municipal Loans | Pass
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance		353,370		344,366
Municipal Loans | Pass Watch
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	0		0
Municipal Loans | Special Mention
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	0		930
Municipal Loans | Substandard
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	0		502
Municipal Loans | Doubtful
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	0		0
Loans to Individuals
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance		106,431		135,769
PCI loans		472		1,123
Loans to Individuals | Pass
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance		105,536		134,694
Loans to Individuals | Pass Watch
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	4		20
Loans to Individuals | Special Mention
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	4		102
Loans to Individuals | Substandard
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	678		707
Loans to Individuals | Doubtful
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loan balance	[3]	$ 209		$ 246

[1] At December 31, 2018, PCI totals include approximately $14.0 million in new funds to a borrower that has since been upgraded to a Pass credit.

[2] At December 31, 2018, PCI totals include approximately $14.0 million in new funds to a borrower that has since been upgraded to a Pass credit.

[3] Includes PCI loans comprised of $22,000 pass watch, $859,000 special mention, $3.9 million substandard and $1.2 million doubtful as of December 31, 2018. Includes PCI loans comprised of $362,000 pass watch, $6.0 million special mention, $10.5 million substandard and $925,000 doubtful as of December 31, 2017.