ACCUMULATED OTHER COMPREHENSIVE INCOME - Reclassifications out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized investment gains, excluding impairment losses on securities available-for-sale									$ 7,644	$ 5,338	$ 7,634
Total income tax expense	$ 1,597	$ 1,186	$ (1,725)	$ (1,636)	$ (7,904)	$ (918)	$ (1,959)	$ (6,223)	(578)	(17,004)	(21,494)
Net income	$ 26,184	$ 746	$ 5,504	$ 6,804	$ 21,292	$ 4,129	$ 6,128	$ 14,654	39,238	46,203	50,162
Net reclassification adjustment									9,415	7,586	6,243
Unrealized gains on investments: | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized investment gains, excluding impairment losses on securities available-for-sale									12,809	10,364	7,624
Total income tax expense									(4,483)	(3,628)	(2,668)
Net income									8,326	6,736	4,956
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:									(1,474)	(2,383)	(1,327)
Net reclassification adjustment									(958)	(1,549)
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	3,690	3,307
Net reclassification adjustment									2,047	2,399
Total
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification adjustment for amounts amortized into net periodic pension and postretirement benefit income:									1,676	1,307	1,980
Deferred income tax expense									(587)	(457)	(693)
Net reclassification adjustment									$ 1,089	$ 850	$ 1,287