LOANS AND ALLOWANCE FOR PROBABLE LOAN LOSSES - Allowance for Loan Losses Activity by Portfolio Segment (Details)		3 Months Ended										12 Months Ended
		Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)		Dec. 31, 2017 USD ($)		Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)		Dec. 31, 2017 USD ($)		Dec. 31, 2016 USD ($) Loan
Change in Allowances for Loan Losses [Abstract]
Balance at beginning of period					$ 20,781,000	[1]					$ 17,911,000	$ 20,781,000	[1]	$ 17,911,000		$ 19,736,000
Provision (reversal) for loan losses		$ 2,446,000	$ 975,000	$ 1,281,000	3,735,000		$ 1,271,000		$ 960,000	$ 1,346,000	1,098,000	8,437,000	[2]	4,675,000	[2]	9,780,000	[2]
Loans charged off												(4,246,000)		(3,453,000)		(14,387,000)
Recoveries of loans charged off												2,047,000		1,648,000		2,782,000
Balance at end of period		27,019,000					20,781,000	[1]				27,019,000		20,781,000	[1]	17,911,000
Provision expense on PCI Loans												302,000		0		0
Construction Real Estate Loans
Change in Allowances for Loan Losses [Abstract]
Balance at beginning of period					3,676,000	[1]					4,147,000	3,676,000	[1]	4,147,000		4,350,000
Provision (reversal) for loan losses												(72,000)		(437,000)		(472,000)
Loans charged off												(14,000)		(35,000)		0
Recoveries of loans charged off												7,000		1,000		269,000
Balance at end of period		3,597,000					3,676,000	[1]				3,597,000		3,676,000	[1]	4,147,000
1-4 Family Residential Real Estate Loans
Change in Allowances for Loan Losses [Abstract]
Balance at beginning of period					2,445,000	[1]					2,665,000	2,445,000	[1]	2,665,000		2,595,000
Provision (reversal) for loan losses												1,134,000		65,000		(28,000)
Loans charged off												(91,000)		(304,000)		(43,000)
Recoveries of loans charged off												356,000		19,000		141,000
Balance at end of period		3,844,000					2,445,000	[1]				3,844,000		2,445,000	[1]	2,665,000
Commercial Real Estate Loans
Change in Allowances for Loan Losses [Abstract]
Balance at beginning of period					10,821,000	[1]					7,204,000	10,821,000	[1]	7,204,000		4,577,000
Provision (reversal) for loan losses												3,894,000		3,604,000		2,604,000
Loans charged off												(783,000)		0		0
Recoveries of loans charged off												36,000		13,000		23,000
Balance at end of period		13,968,000					10,821,000	[1]				13,968,000		10,821,000	[1]	7,204,000
Commercial Loans
Change in Allowances for Loan Losses [Abstract]
Balance at beginning of period					2,094,000	[1]					2,263,000	2,094,000	[1]	2,263,000		6,596,000
Provision (reversal) for loan losses												2,392,000		242,000		6,397,000
Loans charged off												(756,000)		(723,000)		(11,396,000)	[3]
Recoveries of loans charged off												244,000		312,000		666,000
Balance at end of period		3,974,000					2,094,000	[1]				3,974,000		2,094,000	[1]	2,263,000
Charge-off of large commercial borrowing relationships	[3]															$ 10,900,000
Number of loans partially charged-off | Loan	[3]															2
Municipal Loans
Change in Allowances for Loan Losses [Abstract]
Balance at beginning of period					860,000	[1]					750,000	860,000	[1]	750,000		$ 725,000
Provision (reversal) for loan losses												(335,000)		110,000		(224,000)
Loans charged off												0		0		0
Recoveries of loans charged off												0		0		249,000
Balance at end of period		525,000					860,000	[1]				525,000		860,000	[1]	750,000
Loans to Individuals
Change in Allowances for Loan Losses [Abstract]
Balance at beginning of period					$ 885,000	[1]					$ 882,000	885,000	[1]	882,000		893,000
Provision (reversal) for loan losses												1,424,000		1,091,000		1,503,000
Loans charged off												(2,602,000)		(2,391,000)		(2,948,000)
Recoveries of loans charged off												1,404,000		1,303,000		1,434,000
Balance at end of period		$ 1,111,000					$ 885,000	[1]				$ 1,111,000		$ 885,000	[1]	$ 882,000

[1] Loans acquired with the Diboll acquisition were measured at fair value on November 30, 2017 with no carryover of allowance for loan loss.

[2] Of the $8.4 million in provision for loan losses for the year ended December 31, 2018, $302,000 related to provision expense on PCI loans. Of the $4.7 million and $9.8 million recorded in provision for loan losses for the year ended December 31, 2017 and 2016, respectively, none related to provision expense on PCI loans

[3] Of the $11.4 million in commercial charge-offs recorded for the year ended December 31, 2016, $10.9 million relates to the charge-off of two large commercial borrowing relationships.