INCOME TAXES (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Components of Deferred Tax Assets [Abstract]
Loss reserve discounting	$ 13,929				$ 15,681				$ 13,929	$ 15,681
Unearned premium reserve limitation	16,310				15,648				16,310	15,648
Other policyholders' funds payable	3,052				3,553				3,052	3,553
Other, net	1,730				1,145				1,730	1,145
Total deferred income tax asset	35,021				36,027				35,021	36,027
Net unrealized holding gains on investment securities	(32,506)				(42,195)				(32,506)	(42,195)
Deferred policy acquisition costs	(14,252)				(13,770)				(14,252)	(13,770)
Retirement benefits	(3,606)				(5,712)				(3,606)	(5,712)
Other, net	(3,686)				(3,004)				(3,686)	(3,004)
Total deferred income tax liability	(54,050)				(64,681)				(54,050)	(64,681)
Net deferred income tax liability	(19,029)				(28,654)				$ (19,029)	$ (28,654)
Income tax expense (benefit) reconciliation [Abstract]
United States federal corporate tax rate (in hundredths)									35.00%	35.00%	35.00%
Computed expected income tax expense									$ 25,079	$ 14,318	$ 21,298
Increases (decreases) in tax resulting from:
Tax-exempt interest income									(2,805)	(3,285)	(3,828)
Dividends received deduction									(1,136)	(828)	(876)
Proration of tax-exempt interest and dividends received deduction									591	617	706
Other, net									(235)	93	34
Total income tax expense	4,028	$ 4,732	$ 3,127	$ 9,607	7,711	$ (346)	$ (744)	$ 4,294	21,494	10,915	17,334
Comprehensive Income Loss Tax [Abstract]
Operations	4,028	$ 4,732	$ 3,127	$ 9,607	7,711	$ (346)	$ (744)	$ 4,294	21,494	10,915	17,334
Change in unrealized holding gains on investment securities									(9,689)	16,146	(11,539)
Change in funded status of retirement benefit plans:
Pension plans									(1,748)	(2,619)	5,498
Postretirement benefit plans									(1,071)	(1,330)	12,103
Comprehensive income tax expense									8,986	23,112	23,396
Provision for uncertain tax positions	$ 0				$ 0				0	0
Interest income related to US federal income taxes									0	1	0
Income Tax Examination, Penalties and Interest Expense									$ 0	$ 0	$ 0