ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Components of Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Before Tax
Other comprehensive income (loss), before tax	$ 15,297	$ (20,104)
Tax Effect
Other comprehensive income, tax	4,059	(5,389)
Net of Tax
Total other comprehensive income (loss), net of tax	11,238	(14,715)
Net unrealized gains on investment securities
Before Tax
Other comprehensive income (loss) before reclassification, before tax	15,024	(20,455)
Reclassification from AOCI, before tax	0	0
Other comprehensive income (loss), before tax	15,024	(20,455)
Tax Effect
Other comprehensive income (loss) before reclassifications, tax	4,028	(5,484)
Reclassification from AOCI, tax	0	0
Other comprehensive income, tax	4,028	(5,484)
Net of Tax
Other comprehensive income (loss), before reclassifications, net of tax	10,996	(14,971)
Reclassification from AOCI, net of tax	0	0
Total other comprehensive income (loss), net of tax	10,996	(14,971)
Amortization of net actuarial loss
Before Tax
Reclassification from AOCI, before tax	263	341
Tax Effect
Reclassification from AOCI, tax	29	93
Net of Tax
Reclassification from AOCI, net of tax	234	248
Amortization of net transition obligation
Before Tax
Reclassification from AOCI, before tax	5	5
Tax Effect
Reclassification from AOCI, tax	1	1
Net of Tax
Reclassification from AOCI, net of tax	4	4
Amortization of prior service cost
Before Tax
Reclassification from AOCI, before tax	5	5
Tax Effect
Reclassification from AOCI, tax	1	1
Net of Tax
Reclassification from AOCI, net of tax	4	4
Defined benefit plans, net
Before Tax
Other comprehensive income (loss), before tax	(273)	(351)
Tax Effect
Other comprehensive income, tax	(31)	(95)
Net of Tax
Other comprehensive income (loss), before reclassifications, net of tax	0	0
Reclassification from AOCI, net of tax	242	256
Total other comprehensive income (loss), net of tax	$ 242	$ 256