CHANGE IN ACCUMULATED OTHER COMPREHENSIVE INCOME - Components of Accumulated Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			$ 1,850,306
Other comprehensive income before reclassifications	$ 3,685	$ 20,608	51,342	$ 38,402
Less: Amounts reclassified from accumulated other comprehensive income (loss)	(1,439)	(176)	(2,063)	(320)
Net current-period other comprehensive income (loss)	2,246	20,432	49,279	38,082
Ending Balance	1,823,669		1,823,669
Net change in investment securities available-for-sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	72,436	2,701	26,927	(14,553)
Other comprehensive income before reclassifications	3,703	19,591	49,739	36,856
Less: Amounts reclassified from accumulated other comprehensive income (loss)	(1,450)	(48)	(1,977)	(59)
Net current-period other comprehensive income (loss)	2,253	19,543	47,762	36,797
Ending Balance	74,689	22,244	74,689	22,244
Net change in investment securities held-to-maturity
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	403	686	468	779
Other comprehensive income before reclassifications	0	0	0	(2)
Less: Amounts reclassified from accumulated other comprehensive income (loss)	(57)	(84)	(122)	(175)
Net current-period other comprehensive income (loss)	(57)	(84)	(122)	(177)
Ending Balance	346	602	346	602
Net change in defined benefit plan
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(3,313)	693	(3,317)	834
Other comprehensive income before reclassifications	7	10	43	(89)
Less: Amounts reclassified from accumulated other comprehensive income (loss)	179	(44)	147	(86)
Net current-period other comprehensive income (loss)	186	(34)	190	(175)
Ending Balance	(3,127)	659	(3,127)	659
Net change in fair value of derivatives used for cash flow hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	1,008	(1,824)	(577)	(2,454)
Other comprehensive income before reclassifications	(25)	1,007	1,560	1,637
Less: Amounts reclassified from accumulated other comprehensive income (loss)	(111)	0	(111)	0
Net current-period other comprehensive income (loss)	(136)	1,007	1,449	1,637
Ending Balance	872	(817)	872	(817)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	70,534	2,256	23,501	(15,394)
Ending Balance	$ 72,780	$ 22,688	$ 72,780	$ 22,688