ACCUMULATED OTHER COMPREHENSIVE INCOME - Reclassifications out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated Other Comprehensive Income Loss [Line Items]
Net realized investment gains (losses)		$ 3,387	$ 1,634	$ 2,760	$ 549
Income tax expense, current		(2,069)	(2,374)	(4,115)	(9,992)
Total reclassification adjustment		3,329	2,228	4,680	3,019
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)		4,702	3,061	6,362	3,890
Income tax expense, current		(1,646)	(1,071)	(2,227)	(1,361)
Total reclassification adjustment		3,056	1,990	4,135	2,529
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]	(369)	(415)	(737)	(844)
Prior service credit	[1]	788	782	1,575	1,599
Total before tax		419	367	838	755
Income tax expense, current		(146)	(129)	(293)	(265)
Total reclassification adjustment		$ 273	$ 238	$ 545	$ 490

[1] These reclassified components of accumulated other comprehensive income are included in the computation of net periodic pension and postretirement benefit income (see note 7, Employee Retirement Plans, for additional details).