ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of change in accounting principle				$ (191,244)
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Change in accumulated other comprehensive income before reclassifications	$ (13,409)	$ 28,589	$ 27,990
Reclassification adjustments from accumulated other comprehensive income	5,867	(638)	628
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	214,865	133,892	128,369
Change in accumulated other comprehensive income before reclassifications	(33,564)	48,467	8,461
Reclassification adjustments from accumulated other comprehensive income	620	(4,152)	(2,938)
Accumulated effect of change in enacted tax rate		36,658
Ending balance	(9,323)	214,865	133,892
Liability for underfunded employee benefit costs
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(46,551)	(24,837)	(47,932)
Change in accumulated other comprehensive income before reclassifications	20,155	(19,878)	19,529
Reclassification adjustments from accumulated other comprehensive income	5,247	3,514	3,566
Accumulated effect of change in enacted tax rate		(5,350)
Ending balance	(21,149)	(46,551)	(24,837)
Accumulated other comprehensive income, net of tax
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of change in accounting principle	(191,244)	0	0
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	168,314	109,055	80,437
Accumulated effect of change in enacted tax rate	0	31,308	0
Ending balance	$ (30,472)	$ 168,314	$ 109,055