ACCUMULATED OTHER COMPREHENSIVE INCOME - Reclassifications out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Accumulated Other Comprehensive Income Loss [Line Items]
Net realized investment gains		$ 7,498	$ (390)	$ 11,555	$ 3,215
Income tax expense, current		(3,081)	1,452	(15,594)	(2,848)
Total reclassification adjustment		477	786	6,003	4,788
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		253	527	7,752	5,320
Income tax expense, current		(89)	(184)	(2,713)	(1,862)
Total reclassification adjustment		164	343	5,039	3,458
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]	(345)	(144)	(996)	(433)
Prior service credit	[1]	827	826	2,480	2,480
Total before tax		482	682	1,484	2,047
Income tax expense, current		(169)	(239)	(520)	(717)
Total reclassification adjustment		$ 313	$ 443	$ 964	$ 1,330

[1] These reclassified components of accumulated other comprehensive income are included in the computation of net periodic pension and postretirement benefit income (see Note 5, Employee Retirement Plans, for additional details).