ACCUMULATED OTHER COMPREHENSIVE LOSS - Summary of Changes in Accumulated Other Comprehensive Income or Loss by Component (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Changes in accumulated other comprehensive income or loss by component, net of tax [Roll Forward]
Beginning balances				$ 149,813	$ 143,748
Cumulative effect of account change	[1]					$ (162)
Balances at January 1, 2018, as adjusted						149,651
Total other comprehensive income (loss)		$ (285)	$ 1,784	(3,579)	3,782
Ending balances		151,780	151,962	151,780	151,962
Accumulated Other Comprehensive Loss
Changes in accumulated other comprehensive income or loss by component, net of tax [Roll Forward]
Beginning balances		(13,836)	(8,756)	(10,340)	(10,754)
Cumulative effect of account change	[1]					(202)
Balances at January 1, 2018, as adjusted						(10,542)
Other comprehensive income before reclassification		(299)	1,772	(3,606)	3,749
Amounts reclassified from accumulated other comprehensive income		14	12	27	33
Total other comprehensive income (loss)		(285)	1,784	(3,579)	3,782
Ending balances		(14,121)	(6,972)	(14,121)	(6,972)
Accumulated Net Investment Gain (Loss) Attributable to Parent
Changes in accumulated other comprehensive income or loss by component, net of tax [Roll Forward]
Beginning balances		(6,924)	(2,379)	(3,415)	(4,356)
Cumulative effect of account change						(202)
Balances at January 1, 2018, as adjusted						(3,617)
Other comprehensive income before reclassification		(299)	1,772	(3,606)	3,749
Amounts reclassified from accumulated other comprehensive income		0	(8)	0	(8)
Total other comprehensive income (loss)		(299)	1,764	(3,606)	3,741
Ending balances		(7,223)	(615)	(7,223)	(615)
Defined Benefit and Other Benefit Plans
Changes in accumulated other comprehensive income or loss by component, net of tax [Roll Forward]
Beginning balances		(6,912)	(6,377)	(6,925)	(6,398)
Cumulative effect of account change						0
Balances at January 1, 2018, as adjusted						$ (6,925)
Other comprehensive income before reclassification		0	0	0	0
Amounts reclassified from accumulated other comprehensive income		14	20	27	41
Total other comprehensive income (loss)		14	20	27	41
Ending balances		$ (6,898)	$ (6,357)	$ (6,898)	$ (6,357)

[1] Due to implementation of ASC 2016-01. See "Adoption of New Accounting Standards" discussion in Note 1.