ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Components of Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Before Tax
Other comprehensive income (loss), before tax	$ (2,744)			$ 6,310			$ 20,664	$ 38,499
Tax Effect
Other comprehensive income, tax	(734)			1,641			5,532	10,179
Net of Tax
Total other comprehensive income (loss), net of tax	(2,010)	$ 6,479	$ 10,663	4,669	$ 12,413	$ 11,238	15,132	28,320
Net unrealized gains on investment securities
Before Tax
Other comprehensive income (loss) before reclassification, before tax	(3,434)			6,027			18,994	37,671
Reclassification from AOCI, before tax	352			(36)			352	(36)
Other comprehensive income (loss), before tax	(3,082)			5,991			19,346	37,635
Tax Effect
Other comprehensive income (loss) before reclassifications, tax	(919)			1,615			5,087	10,098
Reclassification from AOCI, tax	94			(10)			94	(10)
Other comprehensive income, tax	(825)			1,605			5,181	10,088
Net of Tax
Other comprehensive income (loss), before reclassifications, net of tax	(2,515)			4,412			13,907	27,573
Reclassification from AOCI, net of tax	258			(26)			258	(26)
Total other comprehensive income (loss), net of tax	(2,257)			4,386			14,165	27,547
Net actuarial losses arising during the period
Before Tax
Reclassification from AOCI, before tax							427
Tax Effect
Reclassification from AOCI, tax							114
Net of Tax
Reclassification from AOCI, net of tax							313
Amortization of net actuarial loss
Before Tax
Reclassification from AOCI, before tax	330			310			867	837
Tax Effect
Reclassification from AOCI, tax	89			31			230	84
Net of Tax
Reclassification from AOCI, net of tax	241			279			637	753
Amortization of net transition obligation
Before Tax
Reclassification from AOCI, before tax	5			5			14	14
Tax Effect
Reclassification from AOCI, tax	1			3			4	4
Net of Tax
Reclassification from AOCI, net of tax	4			2			10	10
Amortization of prior service cost
Before Tax
Reclassification from AOCI, before tax	3			4			10	13
Tax Effect
Reclassification from AOCI, tax	1			2			3	3
Net of Tax
Reclassification from AOCI, net of tax	2			2			7	10
Defined benefit plans, net
Before Tax
Other comprehensive income (loss), before tax	(338)			(319)			(1,318)	(864)
Tax Effect
Other comprehensive income, tax	(91)			(36)			(351)	(91)
Net of Tax
Other comprehensive income (loss), before reclassifications, net of tax	0			0			313	0
Reclassification from AOCI, net of tax	247			283			654	773
Total other comprehensive income (loss), net of tax	$ 247			$ 283			$ 967	$ 773