DISCLOSURES ABOUT THE FAIR VALUE OF FINANCIAL INSTRUMENTS, Recurring Measurement Inputs (Details) $ in Thousands	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
Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	1,248,072	1,275,016
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
Recurring [Member]
Assets [Abstract]
Short-term investments	33,717	23,613
Recurring [Member] | Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	1,248,072	1,275,016
Recurring [Member] | U.S. treasury
Assets [Abstract]
Fixed maturity securities available-for-sale	7,858	8,078
Recurring [Member] | U.S. government-sponsored agencies
Assets [Abstract]
Fixed maturity securities available-for-sale	290,057	297,949
Recurring [Member] | Obligations of states and political subdivisions
Assets [Abstract]
Fixed maturity securities available-for-sale	282,307	307,536
Recurring [Member] | Commercial mortgage-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	75,314	83,980
Recurring [Member] | Residential mortgage-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	146,725	119,799
Recurring [Member] | Other asset-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	21,329	24,114
Recurring [Member] | Corporate
Assets [Abstract]
Fixed maturity securities available-for-sale	424,482	433,560
Recurring [Member] | Equity Securities [Member]
Assets [Abstract]
Equity investments, at fair value	238,768
Equity securities available-for-sale		228,115
Recurring [Member] | Financial services
Assets [Abstract]
Equity investments, at fair value	48,647
Equity securities available-for-sale		43,522
Recurring [Member] | Information technology
Assets [Abstract]
Equity investments, at fair value	39,711
Equity securities available-for-sale		35,810
Recurring [Member] | Healthcare
Assets [Abstract]
Equity investments, at fair value	33,389
Equity securities available-for-sale		30,595
Recurring [Member] | Consumer staples
Assets [Abstract]
Equity investments, at fair value	13,302
Equity securities available-for-sale		14,127
Recurring [Member] | Consumer discretionary
Assets [Abstract]
Equity investments, at fair value	24,087
Equity securities available-for-sale		20,538
Recurring [Member] | Energy
Assets [Abstract]
Equity investments, at fair value	17,808
Equity securities available-for-sale		16,905
Recurring [Member] | Industrials
Assets [Abstract]
Equity investments, at fair value	23,943
Equity securities available-for-sale		28,489
Recurring [Member] | Other
Assets [Abstract]
Equity investments, at fair value	15,029
Equity securities available-for-sale		16,421
Recurring [Member] | Non-redeemable preferred stocks
Assets [Abstract]
Equity investments, at fair value	18,706
Equity securities available-for-sale		21,708
Recurring [Member] | Investment funds [Member]
Assets [Abstract]
Equity investments, at fair value	4,146
Recurring [Member] | Investments measured at net asset value (NAV)
Assets [Abstract]
Short-term investments	0	0
Recurring [Member] | Investments measured at net asset value (NAV) | Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Investments measured at net asset value (NAV) | U.S. treasury
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Investments measured at net asset value (NAV) | U.S. government-sponsored agencies
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Investments measured at net asset value (NAV) | Obligations of states and political subdivisions
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Investments measured at net asset value (NAV) | Commercial mortgage-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Investments measured at net asset value (NAV) | Residential mortgage-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Investments measured at net asset value (NAV) | Other asset-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Investments measured at net asset value (NAV) | Corporate
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Investments measured at net asset value (NAV) | Equity Securities [Member]
Assets [Abstract]
Equity investments, at fair value	4,146
Equity securities available-for-sale		0
Recurring [Member] | Investments measured at net asset value (NAV) | Financial services
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Investments measured at net asset value (NAV) | Information technology
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Investments measured at net asset value (NAV) | Healthcare
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Investments measured at net asset value (NAV) | Consumer staples
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Investments measured at net asset value (NAV) | Consumer discretionary
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Investments measured at net asset value (NAV) | Energy
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Investments measured at net asset value (NAV) | Industrials
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Investments measured at net asset value (NAV) | Other
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Investments measured at net asset value (NAV) | Non-redeemable preferred stocks
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Investments measured at net asset value (NAV) | Investment funds [Member]
Assets [Abstract]
Equity investments, at fair value	4,146
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Assets [Abstract]
Short-term investments	33,717	23,613
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | U.S. treasury
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | U.S. government-sponsored agencies
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Obligations of states and political subdivisions
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Commercial mortgage-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Residential mortgage-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Other asset-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Corporate
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Equity Securities [Member]
Assets [Abstract]
Equity investments, at fair value	225,009
Equity securities available-for-sale		215,916
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Financial services
Assets [Abstract]
Equity investments, at fair value	48,647
Equity securities available-for-sale		43,519
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Information technology
Assets [Abstract]
Equity investments, at fair value	39,711
Equity securities available-for-sale		35,810
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Healthcare
Assets [Abstract]
Equity investments, at fair value	33,389
Equity securities available-for-sale		30,595
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Consumer staples
Assets [Abstract]
Equity investments, at fair value	13,302
Equity securities available-for-sale		14,127
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Consumer discretionary
Assets [Abstract]
Equity investments, at fair value	24,087
Equity securities available-for-sale		20,538
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Energy
Assets [Abstract]
Equity investments, at fair value	17,808
Equity securities available-for-sale		16,905
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Industrials
Assets [Abstract]
Equity investments, at fair value	23,943
Equity securities available-for-sale		28,489
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Other
Assets [Abstract]
Equity investments, at fair value	15,029
Equity securities available-for-sale		16,421
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Non-redeemable preferred stocks
Assets [Abstract]
Equity investments, at fair value	9,093
Equity securities available-for-sale		9,512
Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Investment funds [Member]
Assets [Abstract]
Equity investments, at fair value	0
Recurring [Member] | Significant other observable inputs (Level 2) [Member]
Assets [Abstract]
Short-term investments	0	0
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	1,247,723	1,274,396
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | U.S. treasury
Assets [Abstract]
Fixed maturity securities available-for-sale	7,858	8,078
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | U.S. government-sponsored agencies
Assets [Abstract]
Fixed maturity securities available-for-sale	290,057	297,949
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Obligations of states and political subdivisions
Assets [Abstract]
Fixed maturity securities available-for-sale	282,307	307,536
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Commercial mortgage-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	75,314	83,980
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Residential mortgage-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	146,725	119,799
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Other asset-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	21,329	24,114
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Corporate
Assets [Abstract]
Fixed maturity securities available-for-sale	424,133	432,940
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Equity Securities [Member]
Assets [Abstract]
Equity investments, at fair value	9,613
Equity securities available-for-sale		10,196
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Financial services
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Information technology
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Healthcare
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Consumer staples
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Consumer discretionary
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Energy
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Industrials
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Other
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Non-redeemable preferred stocks
Assets [Abstract]
Equity investments, at fair value	9,613
Equity securities available-for-sale		10,196
Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Investment funds [Member]
Assets [Abstract]
Equity investments, at fair value	0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member]
Assets [Abstract]
Short-term investments	0	0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Debt Securities [Member]
Assets [Abstract]
Fixed maturity securities available-for-sale	$ 349	$ 620
Number of securities categorized as Level 3 fair value measurements | Security	2	2
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | U.S. treasury
Assets [Abstract]
Fixed maturity securities available-for-sale	$ 0	$ 0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | U.S. government-sponsored agencies
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Obligations of states and political subdivisions
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Commercial mortgage-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Residential mortgage-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Other asset-backed
Assets [Abstract]
Fixed maturity securities available-for-sale	0	0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Corporate
Assets [Abstract]
Fixed maturity securities available-for-sale	349	620
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Equity Securities [Member]
Assets [Abstract]
Equity investments, at fair value	$ 0
Equity securities available-for-sale		$ 2,003
Number of securities categorized as Level 3 fair value measurements | Security	1	2
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Financial services
Assets [Abstract]
Equity investments, at fair value	$ 0
Equity securities available-for-sale		$ 3
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Information technology
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Healthcare
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Consumer staples
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Consumer discretionary
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Energy
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Industrials
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Other
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		0
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Non-redeemable preferred stocks
Assets [Abstract]
Equity investments, at fair value	0
Equity securities available-for-sale		2,000
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Investment funds [Member]
Assets [Abstract]
Equity investments, at fair value	0
Not Reported At Fair Value [Member]
Liabilities [Abstract]
Surplus notes	15,145	16,689
Not Reported At Fair Value [Member] | Investments measured at net asset value (NAV)
Liabilities [Abstract]
Surplus notes	0	0
Not Reported At Fair Value [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Liabilities [Abstract]
Surplus notes	0	0
Not Reported At Fair Value [Member] | Significant other observable inputs (Level 2) [Member]
Liabilities [Abstract]
Surplus notes	0	0
Not Reported At Fair Value [Member] | Significant unobservable inputs (Level 3) [Member]
Liabilities [Abstract]
Surplus notes	$ 15,145	$ 16,689