CHANGE IN ACCUMULATED OTHER COMPREHENSIVE INCOME - Components of Accumulated Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			$ 1,850,306
Other comprehensive income (loss) before reclassifications	$ (3,425)	$ 9,627	47,899	$ 47,996
Less: Amounts reclassified from accumulated other comprehensive (loss) income	(2,760)	(96)	(4,805)	(383)
Net current-period other comprehensive income (loss)	(6,185)	9,531	43,094	47,613
Ending Balance	1,863,499		1,863,499
Net change in investment securities available-for-sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	74,689	22,244	26,927	(14,553)
Other comprehensive income (loss) before reclassifications	(3,474)	9,268	46,264	46,124
Less: Amounts reclassified from accumulated other comprehensive (loss) income	(2,525)	0	(4,501)	(59)
Net current-period other comprehensive income (loss)	(5,999)	9,268	41,763	46,065
Ending Balance	68,690	31,512	68,690	31,512
Net change in investment securities held-to-maturity
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	346	602	468	779
Other comprehensive income (loss) before reclassifications	0	(1)	0	(2)
Less: Amounts reclassified from accumulated other comprehensive (loss) income	(39)	(69)	(161)	(245)
Net current-period other comprehensive income (loss)	(39)	(70)	(161)	(247)
Ending Balance	307	532	307	532
Net change in defined benefit plan
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(3,127)	659	(3,317)	834
Other comprehensive income (loss) before reclassifications	58	(8)	84	(131)
Less: Amounts reclassified from accumulated other comprehensive (loss) income	(83)	(27)	81	(79)
Net current-period other comprehensive income (loss)	(25)	(35)	165	(210)
Ending Balance	(3,152)	624	(3,152)	624
Net change in fair value of derivatives used for cash flow hedges (1)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	872	(817)	(577)	(2,454)
Other comprehensive income (loss) before reclassifications	0	368	1,560	2,005
Less: Amounts reclassified from accumulated other comprehensive (loss) income	(113)	0	(224)	0
Net current-period other comprehensive income (loss)	(113)	368	1,336	2,005
Ending Balance	759	(449)	759	(449)
Net change in equity method investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	0	0	0	0
Other comprehensive income (loss) before reclassifications	(9)	0	(9)	0
Less: Amounts reclassified from accumulated other comprehensive (loss) income	0	0	0	0
Net current-period other comprehensive income (loss)	(9)	0	(9)	0
Ending Balance	(9)	0	(9)	0
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	72,780	22,688	23,501	(15,394)
Ending Balance	$ 66,595	$ 32,219	$ 66,595	$ 32,219