ACCUMULATED OTHER COMPREHENSIVE INCOME - Changes in Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 704,906	$ 657,184
Net current period other comprehensive (loss) income	(109,780)	(12,634)
Ending balance	609,102	651,668
Total
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	16,093	27,744
Other comprehensive loss before reclassification	(109,807)	(12,084)
Amounts reclassified from accumulated other comprehensive income	27	(550)
Net current period other comprehensive (loss) income	(109,780)	(12,634)
Ending balance	(93,687)	15,110
Unrealized Gains and Losses on Available- for-Sales Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	17,056	29,182
Other comprehensive loss before reclassification	(109,807)	(12,084)
Amounts reclassified from accumulated other comprehensive income	0	(595)
Net current period other comprehensive (loss) income	(109,807)	(12,679)
Ending balance	(92,751)	16,503
Defined Benefit Pension Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(963)	(1,438)
Other comprehensive loss before reclassification	0	0
Amounts reclassified from accumulated other comprehensive income	27	45
Net current period other comprehensive (loss) income	27	45
Ending balance	$ (936)	$ (1,393)