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	Jun. 30, 2018	Dec. 31, 2017
Fair value [Abstract]
Less than 12 months		$ 207,425
12 months or longer		31,440
Total		238,865
Gross unrealized losses [Abstract]
Less than 12 months		1,669
12 months or longer		610
Total		2,279
Debt Securities [Member]
Fair value [Abstract]
Less than 12 months	$ 322,273	203,113
12 months or longer	28,550	31,440
Total	350,823	234,553
Gross unrealized losses [Abstract]
Less than 12 months	6,639	1,390
12 months or longer	1,003	610
Total	7,642	2,000
United States Government Obligations and Authorities [Member]
Fair value [Abstract]
Less than 12 months	61,943	52,368
12 months or longer	20,748	19,287
Total	82,691	71,655
Gross unrealized losses [Abstract]
Less than 12 months	1,300	517
12 months or longer	628	329
Total	1,928	846
Obligations of States and Political Subdivisions [Member]
Fair value [Abstract]
Less than 12 months	14,750	32,030
12 months or longer	1,809	5,676
Total	16,559	37,706
Gross unrealized losses [Abstract]
Less than 12 months	190	221
12 months or longer	88	157
Total	278	378
Corporate [Member]
Fair value [Abstract]
Less than 12 months	229,770	109,780
12 months or longer	5,993	6,452
Total	235,763	116,232
Gross unrealized losses [Abstract]
Less than 12 months	4,955	625
12 months or longer	287	124
Total	5,242	749
International [Member]
Fair value [Abstract]
Less than 12 months	15,810	8,935
12 months or longer	0	25
Total	15,810	8,960
Gross unrealized losses [Abstract]
Less than 12 months	194	27
12 months or longer	0	0
Total	$ 194	27
Equity Securities [Member]
Fair value [Abstract]
Less than 12 months		4,312
12 months or longer		0
Total		4,312
Gross unrealized losses [Abstract]
Less than 12 months		279
12 months or longer		0
Total		$ 279