ACCUMULATED OTHER COMPREHENSIVE INCOME - Reclassifications out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income Loss [Line Items]
Net realized investment gains		$ 4,074	$ 6,153	$ 4,349
Income tax expense, current		(18,061)	(18,611)	(7,280)
Total reclassification adjustment		7,586	6,243	6,451
Unrealized Gains and (Losses) on Available-for-sale securities | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Net realized investment gains		10,364	7,624	7,195
Income tax expense, current		(3,628)	(2,668)	(2,518)
Total reclassification adjustment		6,736	4,956	4,677
Unrecognized pension and postretirement benefit obligations | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Net actuarial loss	[1]	(2,383)	(1,327)	(578)
Prior service credit	[1]	3,690	3,307	3,307
Total before tax		1,307	1,980	2,729
Income tax expense, current		(457)	(693)	(955)
Total reclassification adjustment		$ 850	$ 1,287	$ 1,774

[1] These reclassified components of accumulated other comprehensive income are included in the computation of net periodic pension and postretirement benefit income (see note 12, Employee Retirement Plans, for additional details).