ACCUMULATED OTHER COMPREHENSIVE INCOME - Reclassifications out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized investment gains/losses and change in unrealized gains on equity investments		$ 4,258	$ (5,860)	$ 26,901	$ (11,253)
Total income tax expense (benefit)		(356)	2,727	(9,110)	3,179
Net income (loss)		1,285	(4,995)	34,816	(5,071)
Net reclassification adjustment		366	(3,800)	464	(3,452)
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		50	(5,490)	(222)	(5,734)
Total income tax expense (benefit)		(10)	1,153	47	1,204
Net income (loss)		40	(4,337)	(175)	(4,530)
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]	(234)	(107)	(483)	(210)
Net reclassification adjustment				(382)	(166)
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	787	1,292	1,575
Net reclassification adjustment				1,021	1,244
Total
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification adjustment for amounts amortized into net periodic pension and postretirement benefit income:		412	680	809	1,365
Deferred income tax (expense) benefit		(86)	(143)	(170)	(287)
Net reclassification adjustment		$ 326	$ 537	$ 639	$ 1,078

[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).