ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
	Jan. 01, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 804,479	$ 1,537,121	$ 1,371,896	$ 1,456,347	$ 804,479
Reclassification for gains included in net income, net of tax		0	(41)	(94)	(70)
Current year changes, net of tax				20,402	(14,834)
Adjusted balance, January 1, 2018	(1,672)
Balance at end of period		2,420,723	1,404,977	2,420,723	1,404,977
Unrealized Gain (Loss) on Derivatives
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	292			351	292
Reclassification for gains included in net income, net of tax				0	0
Current year changes, net of tax				(505)	347
Reclassification to retained earnings due to change in federal corporate tax rate	(53)
Adjusted balance, January 1, 2018	239
Balance at end of period		(154)	586	(154)	586
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 for gains included in net income, net of tax				(94)	(70)
Current year changes, net of tax				20,907	(15,181)
Reclassification to retained earnings due to change in federal corporate tax rate	(339)
Adjusted balance, January 1, 2018	(1,911)
Balance at end of period		15,636	(17,162)	15,636	(17,162)
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(1,280)	16,462	(12,571)	(4,826)	(1,280)
Reclassification to retained earnings due to change in federal corporate tax rate	$ (392)				(392)
Balance at end of period		$ 15,482	$ (16,576)	$ 15,482	$ (16,576)