CHANGE IN ACCUMULATED OTHER COMPREHENSIVE LOSS - Components of Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands		3 Months Ended	6 Months Ended
Mar. 31, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities transactions		$ 0	$ 708	$ 21	$ 1,028
Income taxes		(6,907)	(10,850)	(17,676)	(19,440)
Income before taxes		35,647	31,420	83,766	58,947
Net income		28,740	20,570	66,090	39,507
Prior service cost (credit), tax adjustment	$ 100
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income		(155)	(574)	(231)	(904)
Reclassification out of Accumulated Other Comprehensive Income \| Net change in defined benefit plan
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Prior service costs (credits)		(19)	(19)	40	(38)
Transition obligation		0	0	0	0
Actuarial gains		(12)	(18)	(23)	(34)
Income taxes		(7)	15	4	27
Income before taxes		(31)	(37)	17	(72)
Net income		(38)	(22)	21	(45)
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			(708)	(21)	(1,028)
Income taxes		0	253	5	367
Net income		0	(455)	(16)	(661)
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]
Amortization of net unrealized gains to income during the period		(153)	(159)	(309)	(319)
Income taxes		36	62	73	121
Net income		$ (117)	$ (97)	$ (236)	$ (198)

Amount of unrealized and realized gain (loss) on investment in debt and equity securities.

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