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, 2018	Dec. 31, 2017
Fair value [Abstract]
Less than 12 months	$ 201,896	$ 203,113
12 months or longer	92,423	31,440
Total	294,319	234,553
Gross unrealized losses [Abstract]
Less than 12 months	3,539	1,390
12 months or longer	3,124	610
Total	6,663	2,000
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	874	734
12 months or longer	3,203	2,730
Total	4,077	3,464
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Less than 12 months	8	4
12 months or longer	157	102
Total	165	106
Equity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value		4,312
Equity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		279
Equity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value		0
Equity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0
Equity Securities, Unrealized Loss Position, Fair Value		4,312
Equity Securities, Unrealized Loss Position, Accumulated Loss		279
Less than 12 Months, Fair Value	202,770	208,159
Less than 12 Months, Accumulated Loss	3,547	1,673
12 Months or Longer, Fair Value	95,626	34,170
12 Months or Longer, Accumulated Loss	3,281	712
Fair Value	298,396	242,329
Accumulated Loss	6,828	2,385
United States Government Obligations and Authorities [Member]
Fair value [Abstract]
Less than 12 months	22,673	52,368
12 months or longer	29,727	19,287
Total	52,400	71,655
Gross unrealized losses [Abstract]
Less than 12 months	246	517
12 months or longer	905	329
Total	1,151	846
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	229	523
12 months or longer	3,113	2,730
Total	3,342	3,253
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Less than 12 months	1	4
12 months or longer	157	102
Total	158	106
Obligations of States and Political Subdivisions [Member]
Fair value [Abstract]
Less than 12 months	3,254	32,030
12 months or longer	4,786	5,676
Total	8,040	37,706
Gross unrealized losses [Abstract]
Less than 12 months	18	221
12 months or longer	112	157
Total	130	378
Corporate [Member]
Fair value [Abstract]
Less than 12 months	160,361	109,780
12 months or longer	53,232	6,452
Total	213,593	116,232
Gross unrealized losses [Abstract]
Less than 12 months	3,058	625
12 months or longer	1,913	124
Total	4,971	749
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	591	211
12 months or longer	90	0
Total	681	211
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Less than 12 months	6	0
12 months or longer	0	0
Total	6	0
International [Member]
Fair value [Abstract]
Less than 12 months	15,608	8,935
12 months or longer	4,678	25
Total	20,286	8,960
Gross unrealized losses [Abstract]
Less than 12 months	217	27
12 months or longer	194	0
Total	411	$ 27
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months	54
12 months or longer	0
Total	54
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Less than 12 months	1
12 months or longer	0
Total	$ 1