FEDERAL INCOME TAX (Net Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax liabilities
Deferred policy acquisition costs	$ 19,801	$ 19,487
Investments in partnerships	2,156	2,510
Prepaid pension cost	4,441	4,158
Net bond discount accretion	357	296
Depreciation	1,908	1,063
Revaluation of investment basis	377	419
Identifiable intangible assets	1,540	1,689
Other	1,416	1,639
Gross deferred tax liability	93,216	67,213
Deferred tax assets
Financial statement reserves in excess of income tax reserves	20,845	19,800
Unearned premium adjustment	20,816	20,406
Net operating loss carryforwards	1,708	0
Underfunded benefit plan obligation	9,072	5,622
Post-retirement benefits other than pensions	10,785	12,035
Other-than-temporary impairment of investments	2,094	2,094
Contingent ceding commission accrual	0	14
Compensation expense related to stock options	2,394	3,506
Other	4,916	4,648
Deferred tax asset	72,630	68,125
Net deferred tax liability (asset)	20,586
Net deferred tax liability (asset)		(912)
Equity securities
Deferred tax liabilities
Net unrealized appreciation on investment securities	48,652	38,430
All other securities
Deferred tax liabilities
Net unrealized appreciation on investment securities	$ 12,568	$ (2,478)