ACCUMULATED OTHER COMPREHENSIVE INCOME - Reclassifications out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized investment gains (losses) and, beginning in 2018, change in unrealized investment gains on equity investments		$ (5,393)	$ (627)
Total income tax expense (benefit)		452	(1,636)
Net income (loss)		(76)	6,804
Net reclassification adjustment		348	1,351
Unrealized gains (losses) on investments: | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized investment gains (losses) and, beginning in 2018, change in unrealized investment gains on equity investments		(244)	1,660
Total income tax expense (benefit)		51	(581)
Net income (loss)		(193)	1,079
Unrecognized pension and postretirement benefit obligations, net actuarial loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification adjustment for amounts amortized into net periodic pension and postretirement benefit income:	[1]	(103)	(368)
Net reclassification adjustment		(81)	(239)
Unrecognized pension and postretirement benefit obligations, prior service credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification adjustment for amounts amortized into net periodic pension and postretirement benefit income:	[1]	788	787
Net reclassification adjustment		622	511
Total
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification adjustment for amounts amortized into net periodic pension and postretirement benefit income:		685	419
Deferred income tax (expense) benefit		(144)	(147)
Net reclassification adjustment		$ 541	$ 272

[1] These reclassified components of accumulated other comprehensive income are included in the computation of net periodic pension and postretirement benefit income (see note 7, Employee Retirement Plans, for additional details).