ACCUMULATED OTHER COMPREHENSIVE INCOME - Reclassifications out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Accumulated Other Comprehensive Income Loss [Line Items]
Net realized investment gains (losses)		$ (1,192)	$ 7,498	$ (643)	$ 11,555
Income tax expense, current		(1,448)	(3,081)	(11,440)	(15,594)
Total reclassification adjustment		583	477	3,602	6,003
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 (losses)		718	253	4,608	7,752
Income tax expense, current		(252)	(89)	(1,613)	(2,713)
Total reclassification adjustment		466	164	2,995	5,039
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]	(1,112)	(345)	(1,956)	(996)
Prior service credit	[1]	1,291	827	2,890	2,480
Total before tax		179	482	934	1,484
Income tax expense, current		(62)	(169)	(327)	(520)
Total reclassification adjustment		$ 117	$ 313	$ 607	$ 964

[1] These reclassified components of accumulated other comprehensive income are included in the computation of net periodic pension and postretirement benefit income (see Note 6, Employee Retirement Plans, for additional details).