LOANS AND ALLOWANCE FOR PROBABLE LOAN LOSSES - Nonperforming Assets by Asset Class (Details) $ in Thousands		3 Months Ended	12 Months Ended
		Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Nonperforming Assets by Asset Class [Abstract]
Nonaccrual loans	[1],[2]	$ 35,770	$ 35,770	$ 2,937
Accruing loans past due more than 90 days	[1]	0	0	1
Restructured loans	[3]	5,930	5,930	5,767
Other real estate owned		1,206	1,206	1,613
Repossessed assets		0	0	154
Total Nonperforming Assets		$ 42,906	$ 42,906	10,472
Number of commercial real estate loans added to nonaccrual		1	4
Restructured nonaccrual loans		$ 10,900	$ 10,900	1,300
PCI loans restructured		$ 3,100	$ 3,100	$ 2,900

[1] Excludes 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. The increase in nonaccrual loans was primarily due to the addition of four commercial real estate loans to nonaccrual status during the year, one of which was added during the fourth quarter.

[2] Includes $10.9 million and $1.3 million of restructured loans as of December 31, 2018 and 2017, respectively.

[3] Includes $3.1 million and $2.9 million in PCI loans restructured as of December 31, 2018 and 2017, respectively.