CHANGE IN ACCUMULATED OTHER COMPREHENSIVE LOSS - Components of Accumulated Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 820,920
Other comprehensive (loss) income before reclassifications	17,794	$ (12,592)
Less: Amounts reclassified from accumulated other comprehensive (loss) income	(144)	(76)
Net current-period other comprehensive income (loss)	17,650	(12,668)
Ending Balance	1,789,752
Net change in investment securities available for sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(14,553)	(7,842)
Other comprehensive (loss) income before reclassifications	17,265	(11,827)
Less: Amounts reclassified from accumulated other comprehensive (loss) income	(11)	(16)
Net current-period other comprehensive income (loss)	17,254	(11,843)
Ending Balance	2,701	(19,685)
Net change in investment securities held to maturity
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	779	1,223
Other comprehensive (loss) income before reclassifications	(2)	0
Less: Amounts reclassified from accumulated other comprehensive (loss) income	(91)	(119)
Net current-period other comprehensive income (loss)	(93)	(119)
Ending Balance	686	1,104
Net change in defined benefit plan
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	834	865
Other comprehensive (loss) income before reclassifications	(99)	0
Less: Amounts reclassified from accumulated other comprehensive (loss) income	(42)	59
Net current-period other comprehensive income (loss)	(141)	59
Ending Balance	693	924
Net change in fair value of derivatives used for cash flow hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(2,454)	(2,398)
Other comprehensive (loss) income before reclassifications	630	(765)
Less: Amounts reclassified from accumulated other comprehensive (loss) income	0	0
Net current-period other comprehensive income (loss)	630	(765)
Ending Balance	(1,824)	(3,163)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(15,394)	(8,152)
Ending Balance	$ 2,256	$ (20,820)

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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