ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Change in accumulated other comprehensive income before reclassifications	$ 28,589	$ 27,990	$ (12,657)
Reclassification adjustments from accumulated other comprehensive income	(638)	628	1,921
Accumulated effect of change in enacted tax rate	31,308
Net unrealized appreciation on investments
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	133,892	128,369	149,623
Change in accumulated other comprehensive income before reclassifications	48,467	8,461	(18,321)
Reclassification adjustments from accumulated other comprehensive income	(4,152)	(2,938)	(2,933)
Accumulated effect of change in enacted tax rate	36,658
Ending balance	214,865	133,892	128,369
Liability for underfunded employee benefit costs
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(24,837)	(47,932)	(58,450)
Change in accumulated other comprehensive income before reclassifications	(19,878)	19,529	5,664
Reclassification adjustments from accumulated other comprehensive income	3,514	3,566	4,854
Accumulated effect of change in enacted tax rate	(5,350)
Ending balance	(46,551)	(24,837)	(47,932)
Accumulated other comprehensive income, net of tax
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	109,055	80,437	91,173
Accumulated effect of change in enacted tax rate	31,308	0	0
Ending balance	$ 168,314	$ 109,055	$ 80,437