DISCLOSURES ABOUT THE FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) Security	Dec. 31, 2017 USD ($) Security
Assets [Abstract]
Fixed maturity securities available-for-sale	$ 1,248,072	$ 1,275,016
Equity investments, at fair value	238,768
Equity securities available-for-sale		228,115
Short-term investments	33,717	23,613
Liabilities [Abstract]
Surplus notes	$ 25,000	$ 25,000
Period for surplus notes included in assumptions	25 years	25 years
Current interest rate on surplus notes	2.73%	1.35%
Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	$ 1,248,072	$ 1,275,016
Number of broker quoted securities | Security	0	0
Number of securities not priced by independent pricing service | Security	6	8
U.S. treasury
Assets [Abstract]
Fixed maturity securities available-for-sale	$ 7,858	$ 8,078
U.S. government-sponsored agencies
Assets [Abstract]
Fixed maturity securities available-for-sale	290,057	297,949
Obligations of states and political subdivisions
Assets [Abstract]
Fixed maturity securities available-for-sale	282,307	307,536
Commercial mortgage-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	75,314	83,980
Residential mortgage-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	146,725	119,799
Other asset-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	21,329	24,114
Corporate
Assets [Abstract]
Fixed maturity securities available-for-sale	424,482	433,560
Equity Securities [Member]
Assets [Abstract]
Equity securities available-for-sale		228,115
Financial services
Assets [Abstract]
Equity securities available-for-sale		43,522
Information technology
Assets [Abstract]
Equity securities available-for-sale		35,810
Healthcare
Assets [Abstract]
Equity securities available-for-sale		30,595
Consumer staples
Assets [Abstract]
Equity securities available-for-sale		14,127
Consumer discretionary
Assets [Abstract]
Equity securities available-for-sale		20,538
Energy
Assets [Abstract]
Equity securities available-for-sale		16,905
Industrials
Assets [Abstract]
Equity securities available-for-sale		28,489
Other
Assets [Abstract]
Equity securities available-for-sale		16,421
Non-redeemable preferred stocks
Assets [Abstract]
Equity securities available-for-sale		21,708
Carrying amount [Member]
Assets [Abstract]
Short-term investments	33,717	23,613
Liabilities [Abstract]
Surplus notes	25,000	25,000
Carrying amount [Member] | Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	1,248,072	1,275,016
Carrying amount [Member] | U.S. treasury
Assets [Abstract]
Fixed maturity securities available-for-sale	7,858	8,078
Carrying amount [Member] | U.S. government-sponsored agencies
Assets [Abstract]
Fixed maturity securities available-for-sale	290,057	297,949
Carrying amount [Member] | Obligations of states and political subdivisions
Assets [Abstract]
Fixed maturity securities available-for-sale	282,307	307,536
Carrying amount [Member] | Commercial mortgage-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	75,314	83,980
Carrying amount [Member] | Residential mortgage-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	146,725	119,799
Carrying amount [Member] | Other asset-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	21,329	24,114
Carrying amount [Member] | Corporate
Assets [Abstract]
Fixed maturity securities available-for-sale	424,482	433,560
Carrying amount [Member] | Equity Securities [Member]
Assets [Abstract]
Equity investments, at fair value	238,768
Equity securities available-for-sale		228,115
Carrying amount [Member] | Financial services
Assets [Abstract]
Equity investments, at fair value	48,647
Equity securities available-for-sale		43,522
Carrying amount [Member] | Information technology
Assets [Abstract]
Equity investments, at fair value	39,711
Equity securities available-for-sale		35,810
Carrying amount [Member] | Healthcare
Assets [Abstract]
Equity investments, at fair value	33,389
Equity securities available-for-sale		30,595
Carrying amount [Member] | Consumer staples
Assets [Abstract]
Equity investments, at fair value	13,302
Equity securities available-for-sale		14,127
Carrying amount [Member] | Consumer discretionary
Assets [Abstract]
Equity investments, at fair value	24,087
Equity securities available-for-sale		20,538
Carrying amount [Member] | Energy
Assets [Abstract]
Equity investments, at fair value	17,808
Equity securities available-for-sale		16,905
Carrying amount [Member] | Industrials
Assets [Abstract]
Equity investments, at fair value	23,943
Equity securities available-for-sale		28,489
Carrying amount [Member] | Other
Assets [Abstract]
Equity investments, at fair value	15,029
Equity securities available-for-sale		16,421
Carrying amount [Member] | Non-redeemable preferred stocks
Assets [Abstract]
Equity investments, at fair value	18,706
Equity securities available-for-sale		21,708
Carrying amount [Member] | Investment funds [Member]
Assets [Abstract]
Equity investments, at fair value	4,146
Estimated fair value [Member]
Assets [Abstract]
Short-term investments	33,717	23,613
Liabilities [Abstract]
Surplus notes	15,145	16,689
Estimated fair value [Member] | Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	1,248,072	1,275,016
Estimated fair value [Member] | U.S. treasury
Assets [Abstract]
Fixed maturity securities available-for-sale	7,858	8,078
Estimated fair value [Member] | U.S. government-sponsored agencies
Assets [Abstract]
Fixed maturity securities available-for-sale	290,057	297,949
Estimated fair value [Member] | Obligations of states and political subdivisions
Assets [Abstract]
Fixed maturity securities available-for-sale	282,307	307,536
Estimated fair value [Member] | Commercial mortgage-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	75,314	83,980
Estimated fair value [Member] | Residential mortgage-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	146,725	119,799
Estimated fair value [Member] | Other asset-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	21,329	24,114
Estimated fair value [Member] | Corporate
Assets [Abstract]
Fixed maturity securities available-for-sale	424,482	433,560
Estimated fair value [Member] | Equity Securities [Member]
Assets [Abstract]
Equity investments, at fair value	238,768
Equity securities available-for-sale		228,115
Estimated fair value [Member] | Financial services
Assets [Abstract]
Equity investments, at fair value	48,647
Equity securities available-for-sale		43,522
Estimated fair value [Member] | Information technology
Assets [Abstract]
Equity investments, at fair value	39,711
Equity securities available-for-sale		35,810
Estimated fair value [Member] | Healthcare
Assets [Abstract]
Equity investments, at fair value	33,389
Equity securities available-for-sale		30,595
Estimated fair value [Member] | Consumer staples
Assets [Abstract]
Equity investments, at fair value	13,302
Equity securities available-for-sale		14,127
Estimated fair value [Member] | Consumer discretionary
Assets [Abstract]
Equity investments, at fair value	24,087
Equity securities available-for-sale		20,538
Estimated fair value [Member] | Energy
Assets [Abstract]
Equity investments, at fair value	17,808
Equity securities available-for-sale		16,905
Estimated fair value [Member] | Industrials
Assets [Abstract]
Equity investments, at fair value	23,943
Equity securities available-for-sale		28,489
Estimated fair value [Member] | Other
Assets [Abstract]
Equity investments, at fair value	15,029
Equity securities available-for-sale		16,421
Estimated fair value [Member] | Non-redeemable preferred stocks
Assets [Abstract]
Equity investments, at fair value	18,706
Equity securities available-for-sale		$ 21,708
Estimated fair value [Member] | Investment funds [Member]
Assets [Abstract]
Equity investments, at fair value	$ 4,146