FEDERAL INCOME TAX (Net Deferred Tax Liabilities) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Deferred tax liabilities
Deferred policy acquisition costs	$ 44,271		$ 47,809
Prepaid pension cost	2,973		2,556
Net bond discount accretion	1,925		2,069
Depreciation	2,667		2,355
Revaluation of investment basis	2,611	[1]	3,595	[1]
Identifiable intangible assets	3,808	[1]	4,056	[1]
Other	8,152		7,530
Gross deferred tax liability	151,667		131,573
Deferred tax assets
Financial statement reserves in excess of income tax reserves	35,800		39,041
Unearned premium adjustment	26,146		23,502
Net operating loss carryforwards	1,813		2,361
Underfunded benefit plan obligation	31,473		15,858
Postretirement benefits other than pensions	14,374		12,342
Other-than-temporary impairment of investments	5,222		5,222
Contingent ceding commission accrual	3,573	[1]	3,726	[1]
Compensation expense related to stock options	4,582		4,062
AMT credit carryforward	0		476
Other	4,738		5,429
Gross deferred tax asset	127,721		112,019
Valuation allowance	(1,813)		(2,361)
Deferred tax asset	125,908		109,658
Net deferred tax liability	25,759		21,915
Equity Securities [Member]
Deferred tax liabilities
Net unrealized appreciation on investment securities	60,952		55,402
All Other Securities [Member]
Deferred tax liabilities
Net unrealized appreciation on investment securities	$ 24,308		$ 6,201

[1] Related to our acquisition of Mercer Insurance Group