FEDERAL INCOME TAX (Net Deferred Tax Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax liabilities
Deferred policy acquisition costs	$ 30,404	$ 52,031
Investments in partnerships	3,653	8,112
Prepaid pension cost	3,416	4,449
Net bond discount accretion	666	1,134
Depreciation	593	854
Revaluation of investment basis	545	1,342
Identifiable intangible assets	1,838	3,311
Other	1,934	2,831
Gross deferred tax liability	101,487	148,415
Deferred tax assets
Financial statement reserves in excess of income tax reserves	17,036	29,174
Unearned premium adjustment	19,389	30,697
Net operating loss carryforwards	329	718
Underfunded benefit plan obligation	12,375	13,374
Post-retirement benefits other than pensions	11,942	20,221
Other-than-temporary impairment of investments	2,313	4,658
Contingent ceding commission accrual	317	2,769
Alternative minimum tax credit carryforward	6,011	1,194
Compensation expense related to stock options	3,289	4,578
Other	4,145	6,162
Gross deferred tax asset	77,146	113,545
Valuation allowance	(329)	(718)
Deferred tax asset	76,817	112,827
Net deferred tax liability	24,670	35,588
Equity securities
Deferred tax liabilities
Net unrealized appreciation on investment securities	50,839	70,640
All other securities
Deferred tax liabilities
Net unrealized appreciation on investment securities	$ 7,599	$ 3,711