DISCLOSURES ABOUT THE FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) Security	Dec. 31, 2018 USD ($) Security
Assets [Abstract]
Fixed maturity securities available-for-sale	$ 1,291,860	$ 1,282,909
Equity investments, at fair value	242,583	215,363
Short-term investments	48,265	28,204
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%	2.73%
Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	$ 1,291,860	$ 1,282,909
Number of broker quoted securities | Security	0	0
Number of securities not priced by independent pricing service | Security	6	6
U.S. treasury
Assets [Abstract]
Fixed maturity securities available-for-sale	$ 8,113	$ 8,021
U.S. government-sponsored agencies
Assets [Abstract]
Fixed maturity securities available-for-sale	309,120	304,479
Obligations of states and political subdivisions
Assets [Abstract]
Fixed maturity securities available-for-sale	276,857	283,651
Commercial mortgage-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	86,664	84,379
Residential mortgage-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	168,765	162,137
Other asset-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	17,719	20,834
Corporate
Assets [Abstract]
Fixed maturity securities available-for-sale	424,622	419,408
Carrying amount [Member]
Assets [Abstract]
Short-term investments	48,265	28,204
Liabilities [Abstract]
Surplus notes	25,000	25,000
Carrying amount [Member] | Equity Securities [Member]
Assets [Abstract]
Equity investments, at fair value	242,583	215,363
Carrying amount [Member] | Common stocks Information technology [Member]
Assets [Abstract]
Equity investments, at fair value	39,346	31,581
Carrying amount [Member] | Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	1,291,860	1,282,909
Carrying amount [Member] | U.S. treasury
Assets [Abstract]
Fixed maturity securities available-for-sale	8,113	8,021
Carrying amount [Member] | U.S. government-sponsored agencies
Assets [Abstract]
Fixed maturity securities available-for-sale	309,120	304,479
Carrying amount [Member] | Obligations of states and political subdivisions
Assets [Abstract]
Fixed maturity securities available-for-sale	276,857	283,651
Carrying amount [Member] | Commercial mortgage-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	86,664	84,379
Carrying amount [Member] | Residential mortgage-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	168,765	162,137
Carrying amount [Member] | Other asset-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	17,719	20,834
Carrying amount [Member] | Corporate
Assets [Abstract]
Fixed maturity securities available-for-sale	424,622	419,408
Carrying amount [Member] | Financial services
Assets [Abstract]
Equity investments, at fair value	47,345	41,839
Carrying amount [Member] | Investment funds [Member]
Assets [Abstract]
Equity investments, at fair value	9,489	7,641
Carrying amount [Member] | Common stocks Healthcare [Member]
Assets [Abstract]
Equity investments, at fair value	34,006	34,571
Carrying amount [Member] | Common stocks Consumer staples [Member]
Assets [Abstract]
Equity investments, at fair value	14,694	13,180
Carrying amount [Member] | Common stocks Consumer discretionary [Member]
Assets [Abstract]
Equity investments, at fair value	28,764	22,765
Carrying amount [Member] | Common stocks Energy [Member]
Assets [Abstract]
Equity investments, at fair value	15,853	13,372
Carrying amount [Member] | Common stocks Industrials [Member]
Assets [Abstract]
Equity investments, at fair value	19,435	19,389
Carrying amount [Member] | Common stocks Other [Member]
Assets [Abstract]
Equity investments, at fair value	15,471	14,371
Carrying amount [Member] | Nonredeemable Preferred Stock [Member]
Assets [Abstract]
Equity investments, at fair value	18,180	16,654
Estimated fair value [Member]
Assets [Abstract]
Short-term investments	48,265	28,204
Liabilities [Abstract]
Surplus notes	16,056	15,259
Estimated fair value [Member] | Equity Securities [Member]
Assets [Abstract]
Equity investments, at fair value	242,583	215,363
Estimated fair value [Member] | Common stocks Information technology [Member]
Assets [Abstract]
Equity investments, at fair value	39,346	31,581
Estimated fair value [Member] | Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	1,291,860	1,282,909
Estimated fair value [Member] | U.S. treasury
Assets [Abstract]
Fixed maturity securities available-for-sale	8,113	8,021
Estimated fair value [Member] | U.S. government-sponsored agencies
Assets [Abstract]
Fixed maturity securities available-for-sale	309,120	304,479
Estimated fair value [Member] | Obligations of states and political subdivisions
Assets [Abstract]
Fixed maturity securities available-for-sale	276,857	283,651
Estimated fair value [Member] | Commercial mortgage-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	86,664	84,379
Estimated fair value [Member] | Residential mortgage-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	168,765	162,137
Estimated fair value [Member] | Other asset-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	17,719	20,834
Estimated fair value [Member] | Corporate
Assets [Abstract]
Fixed maturity securities available-for-sale	424,622	419,408
Estimated fair value [Member] | Financial services
Assets [Abstract]
Equity investments, at fair value	47,345	41,839
Estimated fair value [Member] | Investment funds [Member]
Assets [Abstract]
Equity investments, at fair value	9,489	7,641
Estimated fair value [Member] | Common stocks Healthcare [Member]
Assets [Abstract]
Equity investments, at fair value	34,006	34,571
Estimated fair value [Member] | Common stocks Consumer staples [Member]
Assets [Abstract]
Equity investments, at fair value	14,694	13,180
Estimated fair value [Member] | Common stocks Consumer discretionary [Member]
Assets [Abstract]
Equity investments, at fair value	28,764	22,765
Estimated fair value [Member] | Common stocks Energy [Member]
Assets [Abstract]
Equity investments, at fair value	15,853	13,372
Estimated fair value [Member] | Common stocks Industrials [Member]
Assets [Abstract]
Equity investments, at fair value	19,435	19,389
Estimated fair value [Member] | Common stocks Other [Member]
Assets [Abstract]
Equity investments, at fair value	15,471	14,371
Estimated fair value [Member] | Nonredeemable Preferred Stock [Member]
Assets [Abstract]
Equity investments, at fair value	$ 18,180	$ 16,654