INCOME TAXES (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Components of Deferred Tax Assets [Abstract]
Loss reserve discounting	$ 8,520				$ 7,507				$ 8,520	$ 7,507
Unearned premium reserve limitation	10,915				10,284				10,915	10,284
Other policyholders' funds payable	1,849				2,103				1,849	2,103
Other, net	1,753				2,202				1,753	2,202
Total deferred income tax asset	23,037				22,096				23,037	22,096
Net unrealized holding gains on investment securities	(13,614)				(22,195)				(13,614)	(22,195)
Deferred policy acquisition costs	(9,400)				(8,634)				(9,400)	(8,634)
Retirement benefits	(2,968)				(3,714)				(2,968)	(3,714)
Other, net	(1,963)				(2,573)				(1,963)	(2,573)
Total deferred income tax liability	(27,945)				(37,116)				(27,945)	(37,116)
Deferred Tax Liabilities, Net	(4,908)				(15,020)				$ (4,908)	$ (15,020)
Income tax expense (benefit) reconciliation [Abstract]
United States federal corporate tax rate (in hundredths)									35.00%	35.00%	35.00%
United States federal corporate tax rate enacted for future periods									21.00%
Computed expected income tax expense (benefit)									$ (3,082)	$ 13,936	$ 22,123
Increases (decreases) in tax resulting from:
Deferred income tax benefit from enactment of the TCJA									0	(9,056)	0
Incremental Benefit Of Net Operating Loss Carry Back									(1,690)	0	0
Tax-exempt interest income									(1,183)	(2,715)	(2,803)
Dividends received deduction									(549)	(1,291)	(1,429)
Proration of tax-exempt interest and dividends received deduction									433	601	635
Investment tax credits									(540)	(672)	(1,392)
Other, net									(597)	(225)	(130)
Operations	(9,325)	$ 5,296	$ (2,727)	$ (452)	(1,597)	$ (1,186)	$ 1,725	$ 1,636	(7,208)	578	17,004
Comprehensive Income Loss Tax [Abstract]
Operations	(9,325)	$ 5,296	$ (2,727)	$ (452)	(1,597)	$ (1,186)	$ 1,725	$ 1,636	(7,208)	578	17,004
Change in unrealized holding gains on investment securities available-for-sale									(2,534)	10,205	(5,720)
Change in funded status of retirement benefit plans:
Pension plans									(1,071)	1,694	414
Postretirement benefit plans									(524)	(774)	(1,345)
Comprehensive income tax expense (benefit)									(11,337)	11,703	10,353
Provision for uncertain tax positions	$ 0				$ 0				0	0
Income tax examination, penalties and interest expense									$ 0	$ 0	$ 0