ACCUMULATED OTHER COMPREHENSIVE INCOME/ (LOSS) - Net Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 1,599,004	$ 1,539,032	$ 1,588,142	$ 1,483,768
Other comprehensive loss before reclassification from accumulated other comprehensive loss, net of tax			20,570	(12,510)
Pension termination - final valuation of plan assets				1,531
Reclassifications from accumulated other comprehensive loss to earnings, net of tax			860	7,438
Total other comprehensive income/ (loss)	26,093	(9,867)	21,430	(3,541)
Balance at end of period	1,628,837	1,537,914	1,628,837	1,537,914
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(101,994)	(125,625)	(97,331)	(131,951)
Total other comprehensive income/ (loss)	26,093	(9,867)	21,430	(3,541)
Balance at end of period	(75,901)	(135,492)	(75,901)	(135,492)
Unrealized Gains/(Losses) on Debt Securities Available-for-Sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			(88,169)	(113,513)
Other comprehensive loss before reclassification from accumulated other comprehensive loss, net of tax			20,570	(12,510)
Pension termination - final valuation of plan assets				0
Reclassifications from accumulated other comprehensive loss to earnings, net of tax			0	0
Total other comprehensive income/ (loss)			20,570	(12,510)
Balance at end of period	(67,599)	(126,023)	(67,599)	(126,023)
Defined Benefit Pension Plan
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			0	(8,002)
Other comprehensive loss before reclassification from accumulated other comprehensive loss, net of tax			0	0
Pension termination - final valuation of plan assets				1,531
Reclassifications from accumulated other comprehensive loss to earnings, net of tax			0	6,471
Total other comprehensive income/ (loss)			0	8,002
Balance at end of period	0	0	0	0
Unrealized Losses on Debt Securities Transferred from Available-for-Sale to Held-to-Maturity
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			(9,162)	(10,436)
Other comprehensive loss before reclassification from accumulated other comprehensive loss, net of tax			0	0
Pension termination - final valuation of plan assets				0
Reclassifications from accumulated other comprehensive loss to earnings, net of tax			860	967
Total other comprehensive income/ (loss)			860	967
Balance at end of period	$ (8,302)	$ (9,469)	$ (8,302)	$ (9,469)