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, 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
Fair value [Abstract]
Less than 12 months	$ 304,521	203,113
12 months or longer	64,856	31,440
Total	369,377	234,553
Gross unrealized losses [Abstract]
Less than 12 months	5,530	1,390
12 months or longer	2,668	610
Total	8,198	2,000
United States Government Obligations and Authorities
Fair value [Abstract]
Less than 12 months	83,061	52,368
12 months or longer	27,124	19,287
Total	110,185	71,655
Gross unrealized losses [Abstract]
Less than 12 months	1,450	517
12 months or longer	1,302	329
Total	2,752	846
Obligations of States and Political Subdivisions
Fair value [Abstract]
Less than 12 months	5,879	32,030
12 months or longer	3,265	5,676
Total	9,144	37,706
Gross unrealized losses [Abstract]
Less than 12 months	94	221
12 months or longer	127	157
Total	221	378
Corporate
Fair value [Abstract]
Less than 12 months	202,142	109,780
12 months or longer	34,306	6,452
Total	236,448	116,232
Gross unrealized losses [Abstract]
Less than 12 months	3,701	625
12 months or longer	1,233	124
Total	4,934	749
International
Fair value [Abstract]
Less than 12 months	13,439	8,935
12 months or longer	161	25
Total	13,600	8,960
Gross unrealized losses [Abstract]
Less than 12 months	285	27
12 months or longer	6	0
Total	$ 291	27
Equity Securities
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