ACCUMULATED OTHER COMPREHENSIVE INCOME - Summary of reclassification out of each component of accumulated other comprehensive income (loss) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gain on securities, net	$ 0	$ 14,000	$ 0	$ 14,000
Interest (expense) income	10,906,000	13,162,000	22,847,000	25,101,000
Compensation and employee benefits	(7,065,000)	(7,134,000)	(14,912,000)	(14,194,000)
Income tax expense	0	(3,000)	0	(52,000)
Total reclassification for the period, net of tax	1,825,000	(40,000)	3,034,000	(593,000)
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense	0	0	0	0
Total reclassification for the period, net of tax	(1,348,000)	184,000	(2,354,000)	554,000
Unrealized Gains and (Losses) on Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest (expense) income	1,346,000	(146,000)	2,350,000	(468,000)
Post-Retirement Plans | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Compensation and employee benefits	2,000	(52,000)	4,000	(100,000)
Unrealized loss on securities available-for-sale: | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gain on securities, net	$ 0	$ 14,000	$ 0	$ 14,000