ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Summary of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	$ 1,456,347	$ 804,479	$ 646,437
Adjusted balance, January 1, 2019			(1,672)
Reclassification for gains included in net income, net of tax	(46)	(70)	(24)
Current year changes, net of tax	22,867	(3,084)	(198)
Balance at end of period	2,469,582	1,456,347	804,479
Unrealized Gain (Loss) on Derivatives
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	351	292	176
Reclassification to retained earnings due to change in federal corporate tax rate		(53)
Adjusted balance, January 1, 2019			239
Reclassification for gains included in net income, net of tax	0	0	0
Current year changes, net of tax	(498)	112	116
Balance at end of period	(147)	351	292
Unrealized Gain (Loss) on Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(5,177)	(1,572)	(1,234)
Reclassification to retained earnings due to change in federal corporate tax rate		(339)
Adjusted balance, January 1, 2019			(1,911)
Reclassification for gains included in net income, net of tax	(46)	(70)	(24)
Current year changes, net of tax	23,365	(3,196)	(314)
Balance at end of period	18,142	(5,177)	(1,572)
Accumulated Other Comprehensive Income (Loss), Net of Tax
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(4,826)	(1,280)	(1,058)
Reclassification to retained earnings due to change in federal corporate tax rate		(392)
Balance at end of period	$ 17,995	$ (4,826)	$ (1,280)