INVESTMENTS (Gross Unrealized Losses and Related Fair Values for Debt and Equity Securities, Grouped by Duration of Time in Continuous Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair value [Abstract]
Less than 12 months	$ 147,077	$ 219,575
12 months or longer	12,999	3,984
Total fair value	160,076	223,559
Gross unrealized losses [Abstract]
Less than 12 months	1,271	4,152
12 months or longer	184	70
Total gross unrealized losses	1,455	4,222
Debt Securities [Member]
Fair value [Abstract]
Less than 12 months	143,599	214,874
12 months or longer	12,757	3,550
Total fair value	156,356	218,424
Gross unrealized losses [Abstract]
Less than 12 months	1,004	3,899
12 months or longer	183	35
Total gross unrealized losses	1,187	3,934
United States Government Obligations and Authorities [Member]
Fair value [Abstract]
Less than 12 months	67,924	45,255
12 months or longer	5,820	111
Total fair value	73,744	45,366
Gross unrealized losses [Abstract]
Less than 12 months	527	850
12 months or longer	74	3
Total gross unrealized losses	601	853
Obligations of States and Political Subdivisions [Member]
Fair value [Abstract]
Less than 12 months	15,454	103,724
12 months or longer	2,966	1,007
Total fair value	18,420	104,731
Gross unrealized losses [Abstract]
Less than 12 months	135	2,066
12 months or longer	30	1
Total gross unrealized losses	165	2,067
Corporate [Member]
Fair value [Abstract]
Less than 12 months	58,838	59,970
12 months or longer	3,971	2,427
Total fair value	62,809	62,397
Gross unrealized losses [Abstract]
Less than 12 months	340	864
12 months or longer	79	31
Total gross unrealized losses	419	895
International [Member]
Fair value [Abstract]
Less than 12 months	1,383	5,925
12 months or longer		5
Total fair value	1,383	5,930
Gross unrealized losses [Abstract]
Less than 12 months	2	119
Total gross unrealized losses	2	119
Equity Securities [Member]
Fair value [Abstract]
Less than 12 months	3,478	4,701
12 months or longer	242	434
Total fair value	3,720	5,135
Gross unrealized losses [Abstract]
Less than 12 months	267	253
12 months or longer	1	35
Total gross unrealized losses	$ 268	$ 288