ACCUMULATED OTHER COMPREHENSIVE INCOME - Reclassifications out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized investment gains (losses), excluding impairment losses on securities available-for-sale		$ (239)	$ (917)	$ 3,254	$ 333
Total income tax expense (benefit)		1,186	(918)	(2,175)	(9,100)
Net income		746	4,129	13,054	24,911
Net reclassification adjustment		535	583	5,215	3,602
Unrealized gains on investments: | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized investment gains (losses), excluding impairment losses on securities available-for-sale		405	718	6,767	4,608
Total income tax expense (benefit)		(141)	(252)	(2,368)	(1,613)
Net income		264	466	4,399	2,995
Unrecognized pension and postretirement benefit obligations, net actuarial loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification adjustment for amounts amortized into net periodic pension and postretirement benefit income:	[1]	(370)	(1,112)	(1,107)	(1,956)
Net reclassification adjustment				(719)	(1,271)
Unrecognized pension and postretirement benefit obligations, prior service credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification adjustment for amounts amortized into net periodic pension and postretirement benefit income:	[1]	787	1,291	2,362	2,890
Net reclassification adjustment				1,535	1,878
Total
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification adjustment for amounts amortized into net periodic pension and postretirement benefit income:		417	179	1,255	934
Deferred income tax expense		(146)	(62)	(439)	(327)
Net reclassification adjustment		$ 271	$ 117	$ 816	$ 607

[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).