CHANGE IN ACCUMULATED OTHER COMPREHENSIVE (LOSS) INCOME - Components of Accumulated Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 1,791,726
Other comprehensive income (loss) before reclassifications	(69,316)	$ 47,657
Less: Amounts reclassified from accumulated other comprehensive (loss) income	(393)	(624)
Net current-period other comprehensive income (loss)	(69,709)	47,033
Ending Balance	1,770,641
Net change in investment securities available-for-sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	59,882	26,927
Other comprehensive income (loss) before reclassifications	(69,589)	46,036
Less: Amounts reclassified from accumulated other comprehensive (loss) income	(250)	(527)
Net current-period other comprehensive income (loss)	(69,839)	45,509
Ending Balance	(9,957)	72,436
Net change in investment securities held-to-maturity
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	276	468
Other comprehensive income (loss) before reclassifications	0	0
Less: Amounts reclassified from accumulated other comprehensive (loss) income	(16)	(65)
Net current-period other comprehensive income (loss)	(16)	(65)
Ending Balance	260	403
Net change in defined benefit plan
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(4,788)	(3,317)
Other comprehensive income (loss) before reclassifications	0	36
Less: Amounts reclassified from accumulated other comprehensive (loss) income	(16)	(32)
Net current-period other comprehensive income (loss)	(16)	4
Ending Balance	(4,804)	(3,313)
Net change in fair value of derivatives used for cash flow hedges (1)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	646	(577)
Other comprehensive income (loss) before reclassifications	0	1,585
Less: Amounts reclassified from accumulated other comprehensive (loss) income	(111)	0
Net current-period other comprehensive income (loss)	(111)	1,585
Ending Balance	535	1,008
Net change in equity method investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(9)	0
Other comprehensive income (loss) before reclassifications	273	0
Less: Amounts reclassified from accumulated other comprehensive (loss) income	0	0
Net current-period other comprehensive income (loss)	273	0
Ending Balance	264	0
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	56,007	23,501
Ending Balance	$ (13,702)	$ 70,534