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DISCLOSURES ABOUT THE FAIR VALUE OF FINANCIAL INSTRUMENTS (Details)
$ in Thousands
 9 Months Ended 12 Months Ended
Sep. 30, 2016
USD ($)
Security
Dec. 31, 2015
USD ($)
Security
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure $ 1,456,432 $ 1,367,268
Short-term investments 42,611 38,599
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 9 7
Available-for-sale Securities, Fair Value Disclosure $ 1,237,150 $ 1,161,025
US Treasury Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 8,207 12,589
US Government-sponsored Enterprises Debt Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 237,494 202,666
US States and Political Subdivisions Debt Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 349,912 344,359
Commercial Mortgage Backed Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 41,882 46,108
Residential Mortgage Backed Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 99,509 88,543
Collateralized Debt Obligations [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 28,251 17,844
Corporate Debt Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 471,895 448,916
Equity Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 219,282 206,243
Common stocks Financial services [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 32,419 33,955
Common stocks Information technology [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 30,937 28,102
Common stocks Healthcare [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 27,519 25,894
Common stocks Consumer staples [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 21,066 18,200
Common stocks Consumer discretionary [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 24,046 18,923
Common stocks Energy [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 18,829 21,068
Common stocks Industrials [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 23,694 20,416
Common stocks Other [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 21,322 20,683
Nonredeemable Preferred Stock [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 19,450 19,002
Carrying amount [Member]    
Assets [Abstract]    
Short-term investments 42,611 38,599
Liabilities [Abstract]    
Surplus notes 25,000 25,000
Carrying amount [Member] | Debt Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 1,237,150 1,161,025
Carrying amount [Member] | US Treasury Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 8,207 12,589
Carrying amount [Member] | US Government-sponsored Enterprises Debt Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 237,494 202,666
Carrying amount [Member] | US States and Political Subdivisions Debt Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 349,912 344,359
Carrying amount [Member] | Commercial Mortgage Backed Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 41,882 46,108
Carrying amount [Member] | Residential Mortgage Backed Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 99,509 88,543
Carrying amount [Member] | Collateralized Debt Obligations [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 28,251 17,844
Carrying amount [Member] | Corporate Debt Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 471,895 448,916
Carrying amount [Member] | Equity Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 219,282 206,243
Carrying amount [Member] | Common stocks Financial services [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 32,419 33,955
Carrying amount [Member] | Common stocks Information technology [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 30,937 28,102
Carrying amount [Member] | Common stocks Healthcare [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 27,519 25,894
Carrying amount [Member] | Common stocks Consumer staples [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 21,066 18,200
Carrying amount [Member] | Common stocks Consumer discretionary [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 24,046 18,923
Carrying amount [Member] | Common stocks Energy [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 18,829 21,068
Carrying amount [Member] | Common stocks Industrials [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 23,694 20,416
Carrying amount [Member] | Common stocks Other [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 21,322 20,683
Carrying amount [Member] | Nonredeemable Preferred Stock [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 19,450 19,002
Estimated fair value [Member]    
Assets [Abstract]    
Short-term investments 42,611 38,599
Liabilities [Abstract]    
Surplus notes 12,131 10,823
Estimated fair value [Member] | Debt Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 1,237,150 1,161,025
Estimated fair value [Member] | US Treasury Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 8,207 12,589
Estimated fair value [Member] | US Government-sponsored Enterprises Debt Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 237,494 202,666
Estimated fair value [Member] | US States and Political Subdivisions Debt Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 349,912 344,359
Estimated fair value [Member] | Commercial Mortgage Backed Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 41,882 46,108
Estimated fair value [Member] | Residential Mortgage Backed Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 99,509 88,543
Estimated fair value [Member] | Collateralized Debt Obligations [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 28,251 17,844
Estimated fair value [Member] | Corporate Debt Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 471,895 448,916
Estimated fair value [Member] | Equity Securities [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 219,282 206,243
Estimated fair value [Member] | Common stocks Financial services [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 32,419 33,955
Estimated fair value [Member] | Common stocks Information technology [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 30,937 28,102
Estimated fair value [Member] | Common stocks Healthcare [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 27,519 25,894
Estimated fair value [Member] | Common stocks Consumer staples [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 21,066 18,200
Estimated fair value [Member] | Common stocks Consumer discretionary [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 24,046 18,923
Estimated fair value [Member] | Common stocks Energy [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 18,829 21,068
Estimated fair value [Member] | Common stocks Industrials [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 23,694 20,416
Estimated fair value [Member] | Common stocks Other [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure 21,322 20,683
Estimated fair value [Member] | Nonredeemable Preferred Stock [Member]    
Assets [Abstract]    
Available-for-sale Securities, Fair Value Disclosure $ 19,450 $ 19,002