ACCUMULATED OTHER COMPREHENSIVE INCOME - Reclassifications out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 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		$ (5,860)	$ 3,387	$ (11,253)	$ 2,760
Total income tax expense (benefit)		2,727	(1,725)	3,179	(3,361)
Net income (loss)		(4,995)	5,504	(5,071)	12,308
Net reclassification adjustment		(3,800)	3,329	(3,452)	4,680
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		(5,490)	4,702	(5,734)	6,362
Total income tax expense (benefit)		1,153	(1,646)	1,204	(2,227)
Net income (loss)		(4,337)	3,056	(4,530)	4,135
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]	(107)	(369)	(210)	(737)
Net reclassification adjustment				(166)	(479)
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]	787	788	1,575	1,575
Net reclassification adjustment				1,244	1,024
Total
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification adjustment for amounts amortized into net periodic pension and postretirement benefit income:		680	419	1,365	838
Deferred income tax (expense) benefit		(143)	(146)	(287)	(293)
Net reclassification adjustment		$ 537	$ 273	$ 1,078	$ 545

[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).