ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance, beginning of year	$ 910,472	$ 888,375	$ 973,373
Change in accumulated other comprehensive income before reclassifications	51,753	38,869	(20,123)
Reclassification adjustments from accumulated other comprehensive income	2,005	4,755	5,867
Balance, end of year	825,149	910,472	888,375
Cumulative effect of change in accounting principle
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance, beginning of year	(30)	(514)	0
Balance, end of year		(30)	(514)
Net unrealized appreciation on investments
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance, beginning of year	47,279	(9,323)	214,865
Change in accumulated other comprehensive income before reclassifications	37,173	55,399	(33,564)
Reclassification adjustments from accumulated other comprehensive income	(1,382)	1,203	620
Balance, end of year	83,070	47,279	(9,323)
Net unrealized appreciation on investments | Cumulative effect of change in accounting principle
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance, beginning of year			(191,244)
Liability for underfunded employee benefit costs
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance, beginning of year	(34,127)	(21,149)	(46,551)
Change in accumulated other comprehensive income before reclassifications	14,580	(16,530)	20,155
Reclassification adjustments from accumulated other comprehensive income	3,388	3,552	5,247
Balance, end of year	(16,159)	(34,127)	(21,149)
Liability for underfunded employee benefit costs | Cumulative effect of change in accounting principle
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance, beginning of year			0
Accumulated other comprehensive income, net of tax
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance, beginning of year	13,152	(30,472)	168,314
Change in accumulated other comprehensive income before reclassifications	51,753	38,869	(13,409)
Reclassification adjustments from accumulated other comprehensive income	2,006	4,755	5,867
Balance, end of year	$ 66,911	$ 13,152	(30,472)
Accumulated other comprehensive income, net of tax | Cumulative effect of change in accounting principle
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance, beginning of year			$ (191,244)