FAIR VALUE (Available-for-Sale Financial Instruments Measured at Fair Value) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Debt securities, available-for-sale, at fair value (amortized cost of $406,173 and $473,126, respectively)	$ 411,718	$ 488,210
Equity securities, at fair value	3,106	3,157
Total investments, at fair value	414,824	491,367
United States government obligations and authorities
Investments, Debt and Equity Securities [Abstract]
Debt securities, available-for-sale, at fair value (amortized cost of $406,173 and $473,126, respectively)	123,785	171,775
Obligations of states and political subdivisions
Investments, Debt and Equity Securities [Abstract]
Debt securities, available-for-sale, at fair value (amortized cost of $406,173 and $473,126, respectively)	22,223	22,264
Corporate securities
Investments, Debt and Equity Securities [Abstract]
Debt securities, available-for-sale, at fair value (amortized cost of $406,173 and $473,126, respectively)	236,856	266,528
International securities
Investments, Debt and Equity Securities [Abstract]
Debt securities, available-for-sale, at fair value (amortized cost of $406,173 and $473,126, respectively)	28,854	27,643
Level 1
Investments, Debt and Equity Securities [Abstract]
Debt securities, available-for-sale, at fair value (amortized cost of $406,173 and $473,126, respectively)	30,431	38,511
Equity securities, at fair value	1,854	1,881
Total investments, at fair value	32,285	40,392
Level 1 | United States government obligations and authorities
Investments, Debt and Equity Securities [Abstract]
Debt securities, available-for-sale, at fair value (amortized cost of $406,173 and $473,126, respectively)	30,431	38,511
Level 1 | Obligations of states and political subdivisions
Investments, Debt and Equity Securities [Abstract]
Debt securities, available-for-sale, at fair value (amortized cost of $406,173 and $473,126, respectively)	0	0
Level 1 | Corporate securities
Investments, Debt and Equity Securities [Abstract]
Debt securities, available-for-sale, at fair value (amortized cost of $406,173 and $473,126, respectively)	0	0
Level 1 | International securities
Investments, Debt and Equity Securities [Abstract]
Debt securities, available-for-sale, at fair value (amortized cost of $406,173 and $473,126, respectively)	0	0
Level 2
Investments, Debt and Equity Securities [Abstract]
Debt securities, available-for-sale, at fair value (amortized cost of $406,173 and $473,126, respectively)	381,287	449,699
Equity securities, at fair value	1,252	1,276
Total investments, at fair value	382,539	450,975
Level 2 | United States government obligations and authorities
Investments, Debt and Equity Securities [Abstract]
Debt securities, available-for-sale, at fair value (amortized cost of $406,173 and $473,126, respectively)	93,354	133,264
Level 2 | Obligations of states and political subdivisions
Investments, Debt and Equity Securities [Abstract]
Debt securities, available-for-sale, at fair value (amortized cost of $406,173 and $473,126, respectively)	22,223	22,264
Level 2 | Corporate securities
Investments, Debt and Equity Securities [Abstract]
Debt securities, available-for-sale, at fair value (amortized cost of $406,173 and $473,126, respectively)	236,856	266,528
Level 2 | International securities
Investments, Debt and Equity Securities [Abstract]
Debt securities, available-for-sale, at fair value (amortized cost of $406,173 and $473,126, respectively)	28,854	27,643
Level 3
Investments, Debt and Equity Securities [Abstract]
Debt securities, available-for-sale, at fair value (amortized cost of $406,173 and $473,126, respectively)	0	0
Equity securities, at fair value	0	0
Total investments, at fair value	0	0
Level 3 | United States government obligations and authorities
Investments, Debt and Equity Securities [Abstract]
Debt securities, available-for-sale, at fair value (amortized cost of $406,173 and $473,126, respectively)	0	0
Level 3 | Obligations of states and political subdivisions
Investments, Debt and Equity Securities [Abstract]
Debt securities, available-for-sale, at fair value (amortized cost of $406,173 and $473,126, respectively)	0	0
Level 3 | Corporate securities
Investments, Debt and Equity Securities [Abstract]
Debt securities, available-for-sale, at fair value (amortized cost of $406,173 and $473,126, respectively)	0	0
Level 3 | International securities
Investments, Debt and Equity Securities [Abstract]
Debt securities, available-for-sale, at fair value (amortized cost of $406,173 and $473,126, respectively)	$ 0	$ 0