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DISCLOSURES ABOUT THE FAIR VALUE OF FINANCIAL INSTRUMENTS (Details)
$ in Thousands
 9 Months Ended 12 Months Ended
Sep. 30, 2017
USD ($)
Security
Dec. 31, 2016
USD ($)
Security
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure $ 1,490,059 $ 1,413,538
Short-term investments 25,255 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 $ 1,258,340 $ 1,199,699
US Treasury Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 8,164 7,830
US Government-sponsored Enterprises Debt Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 255,619 239,197
US States and Political Subdivisions Debt Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 336,638 335,757
Commercial Mortgage Backed Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 65,810 37,572
Residential Mortgage Backed Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 103,666 96,434
Collateralized Debt Obligations [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 24,967 26,393
Corporate Debt Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 463,476 456,516
Equity Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 231,719 213,839
Common stocks Financial services [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 40,738 35,122
Common stocks Information technology [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 36,684 30,542
Common stocks Healthcare [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 30,730 24,707
Common stocks Consumer staples [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 15,044 19,100
Common stocks Consumer discretionary [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 21,605 22,321
Common stocks Energy [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 17,196 19,071
Common stocks Industrials [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 28,315 24,245
Common stocks Other [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 16,787 18,384
Nonredeemable Preferred Stock [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 24,620 20,347
Carrying amount [Member]    
Assets [Abstract]    
Short-term investments 25,255 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 1,258,340 1,199,699
Carrying amount [Member] | US Treasury Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 8,164 7,830
Carrying amount [Member] | US Government-sponsored Enterprises Debt Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 255,619 239,197
Carrying amount [Member] | US States and Political Subdivisions Debt Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 336,638 335,757
Carrying amount [Member] | Commercial Mortgage Backed Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 65,810 37,572
Carrying amount [Member] | Residential Mortgage Backed Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 103,666 96,434
Carrying amount [Member] | Collateralized Debt Obligations [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 24,967 26,393
Carrying amount [Member] | Corporate Debt Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 463,476 456,516
Carrying amount [Member] | Equity Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 231,719 213,839
Carrying amount [Member] | Common stocks Financial services [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 40,738 35,122
Carrying amount [Member] | Common stocks Information technology [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 36,684 30,542
Carrying amount [Member] | Common stocks Healthcare [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 30,730 24,707
Carrying amount [Member] | Common stocks Consumer staples [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 15,044 19,100
Carrying amount [Member] | Common stocks Consumer discretionary [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 21,605 22,321
Carrying amount [Member] | Common stocks Energy [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 17,196 19,071
Carrying amount [Member] | Common stocks Industrials [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 28,315 24,245
Carrying amount [Member] | Common stocks Other [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 16,787 18,384
Carrying amount [Member] | Nonredeemable Preferred Stock [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 24,620 20,347
Estimated fair value [Member]    
Assets [Abstract]    
Short-term investments 25,255 39,670
Liabilities [Abstract]    
Surplus notes 11,512 11,228
Estimated fair value [Member] | Debt Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 1,258,340 1,199,699
Estimated fair value [Member] | US Treasury Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 8,164 7,830
Estimated fair value [Member] | US Government-sponsored Enterprises Debt Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 255,619 239,197
Estimated fair value [Member] | US States and Political Subdivisions Debt Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 336,638 335,757
Estimated fair value [Member] | Commercial Mortgage Backed Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 65,810 37,572
Estimated fair value [Member] | Residential Mortgage Backed Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 103,666 96,434
Estimated fair value [Member] | Collateralized Debt Obligations [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 24,967 26,393
Estimated fair value [Member] | Corporate Debt Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 463,476 456,516
Estimated fair value [Member] | Equity Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 231,719 213,839
Estimated fair value [Member] | Common stocks Financial services [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 40,738 35,122
Estimated fair value [Member] | Common stocks Information technology [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 36,684 30,542
Estimated fair value [Member] | Common stocks Healthcare [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 30,730 24,707
Estimated fair value [Member] | Common stocks Consumer staples [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 15,044 19,100
Estimated fair value [Member] | Common stocks Consumer discretionary [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 21,605 22,321
Estimated fair value [Member] | Common stocks Energy [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 17,196 19,071
Estimated fair value [Member] | Common stocks Industrials [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 28,315 24,245
Estimated fair value [Member] | Common stocks Other [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 16,787 18,384
Estimated fair value [Member] | Nonredeemable Preferred Stock [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure $ 24,620 $ 20,347