ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			$ 804,479
Reclassification to retained earnings due to change in federal corporate tax rate			(392)
Adjusted balance, January 1, 2018					$ (1,672)
Reclassification adjustment for gains on investment securities included in earnings, net of tax of $11, $0, $19 and $13	$ (41)	$ 0	(70)	$ (24)
Current year changes, net of tax			(14,834)	4,323
Balance at end of period	1,404,977	801,921	1,404,977	801,921
Unrealized Gain (Loss) on Derivatives
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			292	176
Reclassification to retained earnings due to change in federal corporate tax rate			(53)
Adjusted balance, January 1, 2018					239
Reclassification adjustment for gains on investment securities included in earnings, net of tax of $11, $0, $19 and $13			0	0
Current year changes, net of tax			347	(38)
Balance at end of period	586	138	586	138
Unrealized Gain (Loss) on Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			(1,572)	(1,234)
Reclassification to retained earnings due to change in federal corporate tax rate			(339)
Adjusted balance, January 1, 2018					(1,911)
Reclassification adjustment for gains on investment securities included in earnings, net of tax of $11, $0, $19 and $13			(70)	(24)
Current year changes, net of tax			(15,181)	4,361
Balance at end of period	(17,162)	3,103	(17,162)	3,103
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			(1,280)	(1,058)
Reclassification to retained earnings due to change in federal corporate tax rate			(392)	0
Adjusted balance, January 1, 2018					$ (1,672)	$ (1,058)
Balance at end of period	$ (16,576)	$ 3,241	$ (16,576)	$ 3,241