ACCUMULATED OTHER COMPREHENSIVE INCOME - Reclassifications out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Accumulated Other Comprehensive Income Loss [Line Items]
Net realized investment gains		$ 3,274	$ 2,343	$ 4,057	$ 3,605
Income tax expense, current		(3,308)	(198)	(12,513)	(4,300)
Total reclassification adjustment		3,755	2,494	5,526	4,002
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		5,317	3,163	7,499	4,793
Income tax expense, current		(1,860)	(1,107)	(2,624)	(1,678)
Total reclassification adjustment		3,457	2,056	4,875	3,115
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]	(367)	(153)	(651)	(289)
Prior service credit	[1]	826	827	1,653	1,654
Total before tax		459	674	1,002	1,365
Income tax expense, current		(161)	(236)	(351)	(478)
Total reclassification adjustment		$ 298	$ 438	$ 651	$ 887

[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).