ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Change in accumulated other comprehensive income before reclassifications	$ 27,990	$ (12,657)	$ 5,332
Reclassification adjustments from accumulated other comprehensive income	628	1,921	(1,309)
Net unrealized appreciation on investments
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	128,369	149,623	116,601
Change in accumulated other comprehensive income before reclassifications	8,461	(18,321)	36,328
Reclassification adjustments from accumulated other comprehensive income	(2,938)	(2,933)	(3,306)
Ending balance	133,892	128,369	149,623
Liability for underfunded employee benefit costs
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(47,932)	(58,450)	(29,451)
Change in accumulated other comprehensive income before reclassifications	19,529	5,664	(30,996)
Reclassification adjustments from accumulated other comprehensive income	3,566	4,854	1,997
Ending balance	(24,837)	(47,932)	(58,450)
Accumulated other comprehensive income, net of tax
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	80,437	91,173	87,150
Change in accumulated other comprehensive income before reclassifications	27,990	(12,657)	5,332
Reclassification adjustments from accumulated other comprehensive income	628	1,921	(1,309)
Ending balance	$ 109,055	$ 80,437	$ 91,173