DISCLOSURES ABOUT THE FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
LIABILITIES
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
Number Of Broker Quoted Securities	1	7
Number of securities not priced by independent pricing service	6	6
Carrying amount [Member]
Assets [Abstract]
Short-term investments	$ 60,317	$ 56,166
Liabilities [Abstract]
Surplus notes	25,000	25,000
Carrying amount [Member] | Debt Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	1,030,001	1,027,984
Carrying amount [Member] | US Treasury Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	9,524	9,412
Carrying amount [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	147,688	146,946
Carrying amount [Member] | US States and Political Subdivisions Debt Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	360,368	357,052
Carrying amount [Member] | Commercial Mortgage Backed Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	59,357	68,939
Carrying amount [Member] | Residential Mortgage Backed Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	94,112	94,179
Carrying amount [Member] | Collateralized Debt Obligations [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	11,525	12,648
Carrying amount [Member] | Corporate Debt Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	347,427	338,808
Carrying amount [Member] | Equity Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	175,674	169,848
Carrying amount [Member] | Common stocks Financial services [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	28,701	28,498
Carrying amount [Member] | Common stocks Information technology [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	23,860	18,917
Carrying amount [Member] | Common stocks Healthcare [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	23,280	21,945
Carrying amount [Member] | Common stocks Consumer staples [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	13,662	13,011
Carrying amount [Member] | Common stocks Consumer discretionary [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	20,960	21,031
Carrying amount [Member] | Common stocks Energy [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	21,527	21,117
Carrying amount [Member] | Common stocks Industrials [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	14,593	17,264
Carrying amount [Member] | Common stocks Other [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	16,919	17,811
Carrying amount [Member] | Nonredeemable Preferred Stock [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	12,172	10,254
Estimated fair value [Member]
Assets [Abstract]
Short-term investments	60,317	56,166
Liabilities [Abstract]
Surplus notes	10,728	10,040
Estimated fair value [Member] | Debt Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	1,030,001	1,027,984
Estimated fair value [Member] | US Treasury Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	9,524	9,412
Estimated fair value [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	147,688	146,946
Estimated fair value [Member] | US States and Political Subdivisions Debt Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	360,368	357,052
Estimated fair value [Member] | Commercial Mortgage Backed Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	59,357	68,939
Estimated fair value [Member] | Residential Mortgage Backed Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	94,112	94,179
Estimated fair value [Member] | Collateralized Debt Obligations [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	11,525	12,648
Estimated fair value [Member] | Corporate Debt Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	347,427	338,808
Estimated fair value [Member] | Equity Securities [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	175,674	169,848
Estimated fair value [Member] | Common stocks Financial services [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	28,701	28,498
Estimated fair value [Member] | Common stocks Information technology [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	23,860	18,917
Estimated fair value [Member] | Common stocks Healthcare [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	23,280	21,945
Estimated fair value [Member] | Common stocks Consumer staples [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	13,662	13,011
Estimated fair value [Member] | Common stocks Consumer discretionary [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	20,960	21,031
Estimated fair value [Member] | Common stocks Energy [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	21,527	21,117
Estimated fair value [Member] | Common stocks Industrials [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	14,593	17,264
Estimated fair value [Member] | Common stocks Other [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	16,919	17,811
Estimated fair value [Member] | Nonredeemable Preferred Stock [Member]
Assets [Abstract]
Available-for-sale Securities, Fair Value Disclosure	$ 12,172	$ 10,254