DISCLOSURES ABOUT THE FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Security	Dec. 31, 2016 USD ($) Security
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	$ 1,503,131	$ 1,413,538
Short-term investments	23,613	39,670
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	1.35%	1.35%
Upward adjustment to fair value input discount rate (percent)	0.50%	0.50%
Period after which rate is adjusted	5 years	5 years
Debt Securities [Member]
Assets [Abstract]
Number Of Broker Quoted Securities | Security	0	0
Number of securities not priced by independent pricing service | Security	8	9
Available-for-sale Securities, Fair Value Disclosure	$ 1,275,016	$ 1,199,699
US Treasury Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	8,078	7,830
US Government-sponsored Enterprises Debt Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	297,949	239,197
US States and Political Subdivisions Debt Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	307,536	335,757
Commercial Mortgage Backed Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	83,980	37,572
Residential Mortgage Backed Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	119,799	96,434
Collateralized Debt Obligations [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	24,114	26,393
Corporate Debt Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	433,560	456,516
Equity Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	228,115	213,839
Common stocks Financial services [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	43,522	35,122
Common stocks Information technology [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	35,810	30,542
Common stocks Healthcare [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	30,595	24,707
Common stocks Consumer staples [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	14,127	19,100
Common stocks Consumer discretionary [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	20,538	22,321
Common stocks Energy [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	16,905	19,071
Common stocks Industrials [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	28,489	24,245
Common stocks Other [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	16,421	18,384
Nonredeemable Preferred Stock [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	21,708	20,347
Carrying amount [Member]
Assets [Abstract]
Short-term investments	23,613	39,670
Liabilities [Abstract]
Surplus notes	25,000	25,000
Carrying amount [Member] | Debt Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	1,275,016	1,199,699
Carrying amount [Member] | US Treasury Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	8,078	7,830
Carrying amount [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	297,949	239,197
Carrying amount [Member] | US States and Political Subdivisions Debt Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	307,536	335,757
Carrying amount [Member] | Commercial Mortgage Backed Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	83,980	37,572
Carrying amount [Member] | Residential Mortgage Backed Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	119,799	96,434
Carrying amount [Member] | Collateralized Debt Obligations [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	24,114	26,393
Carrying amount [Member] | Corporate Debt Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	433,560	456,516
Carrying amount [Member] | Equity Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	228,115	213,839
Carrying amount [Member] | Common stocks Financial services [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	43,522	35,122
Carrying amount [Member] | Common stocks Information technology [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	35,810	30,542
Carrying amount [Member] | Common stocks Healthcare [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	30,595	24,707
Carrying amount [Member] | Common stocks Consumer staples [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	14,127	19,100
Carrying amount [Member] | Common stocks Consumer discretionary [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	20,538	22,321
Carrying amount [Member] | Common stocks Energy [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	16,905	19,071
Carrying amount [Member] | Common stocks Industrials [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	28,489	24,245
Carrying amount [Member] | Common stocks Other [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	16,421	18,384
Carrying amount [Member] | Nonredeemable Preferred Stock [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	21,708	20,347
Estimated fair value [Member]
Assets [Abstract]
Short-term investments	23,613	39,670
Liabilities [Abstract]
Surplus notes	16,689	11,228
Estimated fair value [Member] | Debt Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	1,275,016	1,199,699
Estimated fair value [Member] | US Treasury Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	8,078	7,830
Estimated fair value [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	297,949	239,197
Estimated fair value [Member] | US States and Political Subdivisions Debt Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	307,536	335,757
Estimated fair value [Member] | Commercial Mortgage Backed Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	83,980	37,572
Estimated fair value [Member] | Residential Mortgage Backed Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	119,799	96,434
Estimated fair value [Member] | Collateralized Debt Obligations [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	24,114	26,393
Estimated fair value [Member] | Corporate Debt Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	433,560	456,516
Estimated fair value [Member] | Equity Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	228,115	213,839
Estimated fair value [Member] | Common stocks Financial services [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	43,522	35,122
Estimated fair value [Member] | Common stocks Information technology [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	35,810	30,542
Estimated fair value [Member] | Common stocks Healthcare [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	30,595	24,707
Estimated fair value [Member] | Common stocks Consumer staples [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	14,127	19,100
Estimated fair value [Member] | Common stocks Consumer discretionary [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	20,538	22,321
Estimated fair value [Member] | Common stocks Energy [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	16,905	19,071
Estimated fair value [Member] | Common stocks Industrials [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	28,489	24,245
Estimated fair value [Member] | Common stocks Other [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	16,421	18,384
Estimated fair value [Member] | Nonredeemable Preferred Stock [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	$ 21,708	$ 20,347