FAIR VALUE (Available-for-Sale Financial Instruments Measured at Fair Value) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Debt securities - available-for-sale, at fair value:	$ 424,148	$ 423,238
Equity securities, at fair value	19,535	15,434
Total investments, at fair value	443,683	438,672
United States Government Obligations and Authorities
Investments, Debt and Equity Securities [Abstract]
Debt securities - available-for-sale, at fair value:	114,237	98,137
Obligations of States and Political Subdivisions
Investments, Debt and Equity Securities [Abstract]
Debt securities - available-for-sale, at fair value:	9,679	66,266
Corporate Securities
Investments, Debt and Equity Securities [Abstract]
Debt securities - available-for-sale, at fair value:	283,213	240,919
International
Investments, Debt and Equity Securities [Abstract]
Debt securities - available-for-sale, at fair value:	17,019	17,916
Level 1
Investments, Debt and Equity Securities [Abstract]
Debt securities - available-for-sale, at fair value:	59,689	51,219
Equity securities, at fair value	19,535	15,434
Total investments, at fair value	79,224	66,653
Level 1 | United States Government Obligations and Authorities
Investments, Debt and Equity Securities [Abstract]
Debt securities - available-for-sale, at fair value:	59,689	51,219
Level 1 | Obligations of States and Political Subdivisions
Investments, Debt and Equity Securities [Abstract]
Debt securities - available-for-sale, at fair value:	0
Level 1 | Corporate Securities
Investments, Debt and Equity Securities [Abstract]
Debt securities - available-for-sale, at fair value:	0
Level 1 | International
Investments, Debt and Equity Securities [Abstract]
Debt securities - available-for-sale, at fair value:	0
Level 2
Investments, Debt and Equity Securities [Abstract]
Debt securities - available-for-sale, at fair value:	364,459	372,019
Equity securities, at fair value		0
Total investments, at fair value	364,459	372,019
Level 2 | United States Government Obligations and Authorities
Investments, Debt and Equity Securities [Abstract]
Debt securities - available-for-sale, at fair value:	54,548	46,918
Level 2 | Obligations of States and Political Subdivisions
Investments, Debt and Equity Securities [Abstract]
Debt securities - available-for-sale, at fair value:	9,679	66,266
Level 2 | Corporate Securities
Investments, Debt and Equity Securities [Abstract]
Debt securities - available-for-sale, at fair value:	283,213	240,919
Level 2 | International
Investments, Debt and Equity Securities [Abstract]
Debt securities - available-for-sale, at fair value:	$ 17,019	$ 17,916