CHANGE IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Components of Accumulated Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			$ 820,920
Other comprehensive (loss) income before reclassifications	$ 9,627	$ (6,144)	47,996	$ (23,474)
Less: Amounts reclassified from accumulated other comprehensive (loss) income	(96)	(144)	(383)	(375)
Net current-period other comprehensive income (loss)	9,531	(6,288)	47,613	(23,849)
Ending Balance	1,856,992		1,856,992
Net change in investment securities available for sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	22,244	(24,186)	(14,553)	(7,842)
Other comprehensive (loss) income before reclassifications	9,268	(6,042)	46,124	(22,370)
Less: Amounts reclassified from accumulated other comprehensive (loss) income	0	0	(59)	(16)
Net current-period other comprehensive income (loss)	9,268	(6,042)	46,065	(22,386)
Ending Balance	31,512	(30,228)	31,512	(30,228)
Net change in investment securities held to maturity
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	602	987	779	1,223
Other comprehensive (loss) income before reclassifications	(1)	0	(2)	0
Less: Amounts reclassified from accumulated other comprehensive (loss) income	(69)	(107)	(245)	(343)
Net current-period other comprehensive income (loss)	(70)	(107)	(247)	(343)
Ending Balance	532	880	532	880
Net change in defined benefit plan
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	659	894	834	865
Other comprehensive (loss) income before reclassifications	(8)	7	(131)	15
Less: Amounts reclassified from accumulated other comprehensive (loss) income	(27)	(37)	(79)	(16)
Net current-period other comprehensive income (loss)	(35)	(30)	(210)	(1)
Ending Balance	624	864	624	864
Net change in fair value of derivatives used for cash flow hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(817)	(3,408)	(2,454)	(2,398)
Other comprehensive (loss) income before reclassifications	368	(109)	2,005	(1,119)
Less: Amounts reclassified from accumulated other comprehensive (loss) income	0	0	0	0
Net current-period other comprehensive income (loss)	368	(109)	2,005	(1,119)
Ending Balance	(449)	(3,517)	(449)	(3,517)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	22,688	(25,713)	(15,394)	(8,152)
Ending Balance	$ 32,219	$ (32,001)	$ 32,219	$ (32,001)

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount after tax, before reclassification adjustments, of other comprehensive income (loss), attributable to parent.

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Amount after tax of other comprehensive income (loss) attributable to parent entity.

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Amount after tax of reclassification adjustments of other comprehensive income (loss) attributable to parent.

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