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	Dec. 31, 2017	Dec. 31, 2016
Fair value [Abstract]
Less than 12 months	$ 207,425	$ 219,575
12 months or longer	31,440	3,984
Total fair value	238,865	223,559
Gross unrealized losses [Abstract]
Less than 12 months	1,669	4,152
12 months or longer	610	70
Total gross unrealized losses	2,279	4,222
Debt Securities [Member]
Fair value [Abstract]
Less than 12 months	203,113	214,874
12 months or longer	31,440	3,550
Total fair value	234,553	218,424
Gross unrealized losses [Abstract]
Less than 12 months	1,390	3,899
12 months or longer	610	35
Total gross unrealized losses	2,000	3,934
United States Government Obligations and Authorities [Member]
Fair value [Abstract]
Less than 12 months	52,368	45,255
12 months or longer	19,287	111
Total fair value	71,655	45,366
Gross unrealized losses [Abstract]
Less than 12 months	517	850
12 months or longer	329	3
Total gross unrealized losses	846	853
Obligations of States and Political Subdivisions [Member]
Fair value [Abstract]
Less than 12 months	32,030	103,724
12 months or longer	5,676	1,007
Total fair value	37,706	104,731
Gross unrealized losses [Abstract]
Less than 12 months	221	2,066
12 months or longer	157	1
Total gross unrealized losses	378	2,067
Corporate [Member]
Fair value [Abstract]
Less than 12 months	109,780	59,970
12 months or longer	6,452	2,427
Total fair value	116,232	62,397
Gross unrealized losses [Abstract]
Less than 12 months	625	864
12 months or longer	124	31
Total gross unrealized losses	749	895
International [Member]
Fair value [Abstract]
Less than 12 months	8,935	5,925
12 months or longer	25	5
Total fair value	8,960	5,930
Gross unrealized losses [Abstract]
Less than 12 months	27	119
12 months or longer	0	0
Total gross unrealized losses	27	119
Equity Securities [Member]
Fair value [Abstract]
Less than 12 months	4,312	4,701
12 months or longer	0	434
Total fair value	4,312	5,135
Gross unrealized losses [Abstract]
Less than 12 months	279	253
12 months or longer	0	35
Total gross unrealized losses	$ 279	$ 288