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	Mar. 31, 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	$ 290,005	203,113
12 months or longer	25,242	31,440
Total	315,247	234,553
Gross unrealized losses [Abstract]
Less than 12 months	5,186	1,390
12 months or longer	873	610
Total	6,059	2,000
United States Government Obligations and Authorities [Member]
Fair value [Abstract]
Less than 12 months	61,093	52,368
12 months or longer	18,399	19,287
Total	79,492	71,655
Gross unrealized losses [Abstract]
Less than 12 months	1,284	517
12 months or longer	529	329
Total	1,813	846
Obligations of States and Political Subdivisions [Member]
Fair value [Abstract]
Less than 12 months	15,762	32,030
12 months or longer	1,821	5,676
Total	17,583	37,706
Gross unrealized losses [Abstract]
Less than 12 months	203	221
12 months or longer	84	157
Total	287	378
Corporate [Member]
Fair value [Abstract]
Less than 12 months	198,468	109,780
12 months or longer	5,022	6,452
Total	203,490	116,232
Gross unrealized losses [Abstract]
Less than 12 months	3,547	625
12 months or longer	260	124
Total	3,807	749
International [Member]
Fair value [Abstract]
Less than 12 months	14,682	8,935
12 months or longer	0	25
Total	14,682	8,960
Gross unrealized losses [Abstract]
Less than 12 months	152	27
12 months or longer	0	0
Total	$ 152	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