DISCLOSURES ABOUT THE FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) Security	Dec. 31, 2017 USD ($) Security
Assets [Abstract]
Fixed maturity securities available-for-sale	$ 1,256,473	$ 1,275,016
Equity investments, at fair value	220,498
Equity securities available-for-sale		228,115
Equity investments, at alternative measurement of cost less impairments	2,000	0
Short-term investments	21,664	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	$ 1,256,473	$ 1,275,016
Number of broker quoted securities | Security	0	0
Number of securities not priced by independent pricing service | Security	6	8
US Treasury Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	$ 7,956	$ 8,078
US Government-sponsored Enterprises Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	302,476	297,949
US States and Political Subdivisions Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	293,931	307,536
Commercial Mortgage Backed Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	76,660	83,980
Residential Mortgage Backed Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	132,350	119,799
Collateralized Debt Obligations [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	22,748	24,114
Corporate Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	420,352	433,560
Equity Securities [Member]
Assets [Abstract]
Equity securities available-for-sale		228,115
Common stocks Financial services [Member]
Assets [Abstract]
Equity securities available-for-sale		43,522
Common stocks Information technology [Member]
Assets [Abstract]
Equity securities available-for-sale		35,810
Common stocks Healthcare [Member]
Assets [Abstract]
Equity securities available-for-sale		30,595
Common stocks Consumer staples [Member]
Assets [Abstract]
Equity securities available-for-sale		14,127
Common stocks Consumer discretionary [Member]
Assets [Abstract]
Equity securities available-for-sale		20,538
Common stocks Energy [Member]
Assets [Abstract]
Equity securities available-for-sale		16,905
Common stocks Industrials [Member]
Assets [Abstract]
Equity securities available-for-sale		28,489
Common stocks Other [Member]
Assets [Abstract]
Equity securities available-for-sale		16,421
Nonredeemable Preferred Stock [Member]
Assets [Abstract]
Equity securities available-for-sale		21,708
Carrying amount [Member]
Assets [Abstract]
Short-term investments	21,664	23,613
Liabilities [Abstract]
Surplus notes	25,000	25,000
Carrying amount [Member] | Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	1,256,473	1,275,016
Carrying amount [Member] | US Treasury Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	7,956	8,078
Carrying amount [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	302,476	297,949
Carrying amount [Member] | US States and Political Subdivisions Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	293,931	307,536
Carrying amount [Member] | Commercial Mortgage Backed Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	76,660	83,980
Carrying amount [Member] | Residential Mortgage Backed Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	132,350	119,799
Carrying amount [Member] | Collateralized Debt Obligations [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	22,748	24,114
Carrying amount [Member] | Corporate Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	420,352	433,560
Carrying amount [Member] | Equity Securities [Member]
Assets [Abstract]
Equity investments, at fair value	220,498
Equity securities available-for-sale		228,115
Carrying amount [Member] | Common stocks Financial services [Member]
Assets [Abstract]
Equity investments, at fair value	44,895
Equity securities available-for-sale		43,522
Carrying amount [Member] | Common stocks Information technology [Member]
Assets [Abstract]
Equity investments, at fair value	33,886
Equity securities available-for-sale		35,810
Carrying amount [Member] | Common stocks Healthcare [Member]
Assets [Abstract]
Equity investments, at fair value	28,372
Equity securities available-for-sale		30,595
Carrying amount [Member] | Common stocks Consumer staples [Member]
Assets [Abstract]
Equity investments, at fair value	14,715
Equity securities available-for-sale		14,127
Carrying amount [Member] | Common stocks Consumer discretionary [Member]
Assets [Abstract]
Equity investments, at fair value	21,966
Equity securities available-for-sale		20,538
Carrying amount [Member] | Common stocks Energy [Member]
Assets [Abstract]
Equity investments, at fair value	15,620
Equity securities available-for-sale		16,905
Carrying amount [Member] | Common stocks Industrials [Member]
Assets [Abstract]
Equity investments, at fair value	25,235
Equity securities available-for-sale		28,489
Carrying amount [Member] | Common stocks Other [Member]
Assets [Abstract]
Equity investments, at fair value	15,762
Equity securities available-for-sale		16,421
Carrying amount [Member] | Nonredeemable Preferred Stock [Member]
Assets [Abstract]
Equity investments, at fair value	19,224
Equity securities available-for-sale		21,708
Carrying amount [Member] | Investment funds [Member]
Assets [Abstract]
Equity investments, at fair value	823
Estimated fair value [Member]
Assets [Abstract]
Short-term investments	21,664	23,613
Liabilities [Abstract]
Surplus notes	15,731	16,689
Estimated fair value [Member] | Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	1,256,473	1,275,016
Estimated fair value [Member] | US Treasury Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	7,956	8,078
Estimated fair value [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	302,476	297,949
Estimated fair value [Member] | US States and Political Subdivisions Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	293,931	307,536
Estimated fair value [Member] | Commercial Mortgage Backed Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	76,660	83,980
Estimated fair value [Member] | Residential Mortgage Backed Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	132,350	119,799
Estimated fair value [Member] | Collateralized Debt Obligations [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	22,748	24,114
Estimated fair value [Member] | Corporate Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	420,352	433,560
Estimated fair value [Member] | Equity Securities [Member]
Assets [Abstract]
Equity investments, at fair value	220,498
Equity securities available-for-sale		228,115
Estimated fair value [Member] | Common stocks Financial services [Member]
Assets [Abstract]
Equity investments, at fair value	44,895
Equity securities available-for-sale		43,522
Estimated fair value [Member] | Common stocks Information technology [Member]
Assets [Abstract]
Equity investments, at fair value	33,886
Equity securities available-for-sale		35,810
Estimated fair value [Member] | Common stocks Healthcare [Member]
Assets [Abstract]
Equity investments, at fair value	28,372
Equity securities available-for-sale		30,595
Estimated fair value [Member] | Common stocks Consumer staples [Member]
Assets [Abstract]
Equity investments, at fair value	14,715
Equity securities available-for-sale		14,127
Estimated fair value [Member] | Common stocks Consumer discretionary [Member]
Assets [Abstract]
Equity investments, at fair value	21,966
Equity securities available-for-sale		20,538
Estimated fair value [Member] | Common stocks Energy [Member]
Assets [Abstract]
Equity investments, at fair value	15,620
Equity securities available-for-sale		16,905
Estimated fair value [Member] | Common stocks Industrials [Member]
Assets [Abstract]
Equity investments, at fair value	25,235
Equity securities available-for-sale		28,489
Estimated fair value [Member] | Common stocks Other [Member]
Assets [Abstract]
Equity investments, at fair value	15,762
Equity securities available-for-sale		16,421
Estimated fair value [Member] | Nonredeemable Preferred Stock [Member]
Assets [Abstract]
Equity investments, at fair value	19,224
Equity securities available-for-sale		$ 21,708
Estimated fair value [Member] | Investment funds [Member]
Assets [Abstract]
Equity investments, at fair value	$ 823