CHANGE IN ACCUMULATED OTHER COMPREHENSIVE INCOME - Components of Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities transactions	$ 693	$ 15
Income taxes	(1,288)	(6,260)
Income before taxes	11,855	19,190
Net Income (Loss) Attributable to Parent, Total	10,927	13,023
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Income (Loss) Attributable to Parent, Total	(624)	(144)
Reclassification out of Accumulated Other Comprehensive Income | Net change in defined benefit plan
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income taxes	10	(8)
Prior service costs (credits)	(19)	(19)
Actuarial gains	(23)	(15)
Income before taxes	(42)	(34)
Net Income (Loss) Attributable to Parent, Total	(32)	(42)
Reclassification out of Accumulated Other Comprehensive Income | Realized Gains on Securities Transactions | Net change in investment securities available-for-sale
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities transactions	(693)	(15)
Income taxes	166	4
Net Income (Loss) Attributable to Parent, Total	(527)	(11)
Reclassification out of Accumulated Other Comprehensive Income | Amortization of Net Unrealized Gains to Income During Period | Net change in investment securities available-for-sale
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income taxes	20	29
Amortization of net unrealized gains to income during the period	(85)	(120)
Net Income (Loss) Attributable to Parent, Total	$ (65)	$ (91)