ACCUMULATED OTHER COMPREHENSIVE INCOME - Reclassifications out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized investment gains/losses and change in unrealized gains on equity investments		$ 22,643	$ (5,393)
Total income tax expense (benefit)		(8,754)	452
Net income (loss)		33,531	(76)
Net reclassification adjustment		98	348
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 change in unrealized gains on equity investments		(272)	(244)
Total income tax expense (benefit)		57	51
Net income (loss)		(215)	(193)
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]	(249)	(103)
Net reclassification adjustment		(197)	(81)
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]	646	788
Net reclassification adjustment		510	622
Total
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification adjustment for amounts amortized into net periodic pension and postretirement benefit income:		397	685
Deferred income tax (expense) benefit		(84)	(144)
Net reclassification adjustment		$ 313	$ 541

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