ACCUMULATED OTHER COMPREHENSIVE INCOME - Changes in Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance	$ 657,184	$ 598,100
Net current period other comprehensive income	(12,634)	10,491
Balance	651,668	606,572
Total
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance	27,744	12,059
Other comprehensive income before reclassification	(12,084)	10,444
Amounts reclassified from accumulated other comprehensive income	(550)	47
Net current period other comprehensive income	(12,634)	10,491
Balance	15,110	22,550
Unrealized Gains and Losses on Available-for-Sales Securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance	29,182	13,607
Other comprehensive income before reclassification	(12,084)	10,444
Amounts reclassified from accumulated other comprehensive income	(595)	0
Net current period other comprehensive income	(12,679)	10,444
Balance	16,503	24,051
Defined Benefit Pension Items
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance	(1,438)	(1,548)
Other comprehensive income before reclassification	0	0
Amounts reclassified from accumulated other comprehensive income	45	47
Net current period other comprehensive income	45	47
Balance	$ (1,393)	$ (1,501)