ACCUMULATED OTHER COMPREHENSIVE INCOME - Reclassifications out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total income tax expense (benefit)	$ 9,325	$ (5,296)	$ 2,727	$ 452	$ 1,597	$ 1,186	$ (1,725)	$ (1,636)	$ 7,208	$ (578)	$ (17,004)
Net income (loss)	$ (21,545)	$ 19,148	$ (4,995)	$ (76)	$ 26,184	$ 746	$ 5,504	$ 6,804	(7,468)	39,238	46,203
Net reclassification adjustment									(12,497)	9,415	7,586
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 gains on equity investments									(18,548)	12,809	10,364
Total income tax expense (benefit)									3,895	(4,483)	(3,628)
Net income (loss)									(14,653)	8,326	6,736
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:									(421)	(1,474)	(2,383)
Net reclassification adjustment									(333)	(958)
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:									3,150	3,150	3,690
Net reclassification adjustment									2,489	2,047
Total
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification adjustment for amounts amortized into net periodic pension and postretirement benefit income:									2,729	1,676	1,307
Deferred income tax (expense) benefit									(573)	(587)	(457)
Net reclassification adjustment									$ 2,156	$ 1,089	$ 850