ACCUMULATED OTHER COMPREHENSIVE LOSS (Details) - USD ($) $ in Thousands		3 Months Ended
	Jan. 01, 2018	Mar. 31, 2019	Mar. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 804,479	$ 1,456,347	$ 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 ($12) and ($8)		(46)	(29)
Current year changes, net of tax		3,694	(9,122)
Balance at end of period		1,495,584	868,944
Unrealized Gain (Loss) on Derivatives
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	292	351	292
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 ($12) and ($8)		0	0
Current year changes, net of tax		(173)	281
Balance at end of period		178	520
Unrealized Gain (Loss) on Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(1,572)	(5,177)	(1,572)
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 ($12) and ($8)		(46)	(29)
Current year changes, net of tax		3,867	(9,403)
Balance at end of period		(1,356)	(11,343)
Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ (1,280)	(4,826)	(1,280)
Reclassification to retained earnings due to change in federal corporate tax rate			(392)
Balance at end of period		$ (1,178)	$ (10,823)