INCOME TAXES - Income Tax Expense Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Amount
Statutory tax expense									$ 17,703	$ 24,652	$ 20,886
Increase (decrease) in taxes from:
Tax rate changes									(767)	2,416	0
Tax exempt interest									(6,257)	(10,195)	(9,879)
Bank Owned Life Insurance									(613)	(885)	(915)
Share-based compensation									(191)	(482)	0
Acquisition costs									0	467	0
State Business Tax									297	68	71
Other, net									(9)	80	162
Income tax expense	$ 2,521	$ 2,192	$ 3,360	$ 2,090	$ 5,870	$ 3,890	$ 3,353	$ 3,008	$ 10,163	$ 16,121	$ 10,325
Percent of Pre-Tax Income
Statutory tax expense									21.00%	35.00%	35.00%
Increase (Decrease) in Taxes From:
Percent of Pre-tax income, tax rate changes									(0.90%)	3.40%	0.00%
Percent of Pre-tax income, Tax exempt interest									(7.40%)	(14.50%)	(16.60%)
Percent of Pre-tax income, Bank Owned Life Insurance									(0.70%)	(1.20%)	(1.50%)
Percent of Pre-tax income, Share based compensation									(0.20%)	(0.70%)	0.00%
Percent of Pre-tax income, Acquisition Costs									0.00%	0.70%	0.00%
Percent of Pre-tax income, State Business Tax									0.30%	0.10%	0.10%
Percent of Pre-tax income, Other Net									0.00%	0.10%	0.30%
Percent of Pre-tax income, Income tax expense									12.10%	22.90%	17.30%