INVESTMENTS - Types of Debt and Equity Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt securities
Fair value
Less than 12 months	$ 397,157	$ 372,467
12 months and greater	584,017	254,270
Total Fair Value	981,174	626,737
Cost or amortized Cost
Less than 12 months	406,953	375,790
12 months and greater	603,710	259,335
Total Cost or Amortized Cost	1,010,663	635,125
Unrealized Loss
Less than 12 months	9,796	3,323
12 months and greater	19,693	5,065
Total Unrealized Loss	29,489	8,388
U.S. government
Fair value
Less than 12 months	7,249	58,009
12 months and greater	76,073	30,888
Total Fair Value	83,322	88,897
Cost or amortized Cost
Less than 12 months	7,270	58,443
12 months and greater	77,037	31,349
Total Cost or Amortized Cost	84,307	89,792
Unrealized Loss
Less than 12 months	21	434
12 months and greater	964	461
Total Unrealized Loss	985	895
U.S. Agency
Fair value
Less than 12 months		10,917
12 months and greater	8,843
Total Fair Value	8,843	10,917
Cost or amortized Cost
Less than 12 months		11,027
12 months and greater	9,058
Total Cost or Amortized Cost	9,058	11,027
Unrealized Loss
Less than 12 months		110
12 months and greater	215
Total Unrealized Loss	215	110
Non-U.S. govt. & agency
Fair value
Less than 12 months	5,432
12 months and greater	2,207	1,840
Total Fair Value	7,639	1,840
Cost or amortized Cost
Less than 12 months	5,571
12 months and greater	2,599	1,896
Total Cost or Amortized Cost	8,170	1,896
Unrealized Loss
Less than 12 months	139
12 months and greater	392	56
Total Unrealized Loss	531	56
Mortgage-backed
Fair value
Less than 12 months	25,345	122,130
12 months and greater	261,325	111,306
Total Fair Value	286,670	233,436
Cost or amortized Cost
Less than 12 months	25,486	123,559
12 months and greater	270,632	113,955
Total Cost or Amortized Cost	296,118	237,514
Unrealized Loss
Less than 12 months	141	1,429
12 months and greater	9,307	2,649
Total Unrealized Loss	9,448	4,078
ABS/CMBS
Fair value
Less than 12 months	46,918	23,406
12 months and greater	32,137	21,587
Total Fair Value	79,055	44,993
Cost or amortized Cost
Less than 12 months	47,146	23,491
12 months and greater	32,785	21,817
Total Cost or Amortized Cost	79,931	45,308
Unrealized Loss
Less than 12 months	228	85
12 months and greater	648	230
Total Unrealized Loss	876	315
Corporate Debt
Fair value
Less than 12 months	306,177	86,946
12 months and greater	147,751	28,600
Total Fair Value	453,928	115,546
Cost or amortized Cost
Less than 12 months	315,428	87,736
12 months and greater	154,624	29,491
Total Cost or Amortized Cost	470,052	117,227
Unrealized Loss
Less than 12 months	9,251	790
12 months and greater	6,873	891
Total Unrealized Loss	16,124	1,681
Municipal
Fair value
Less than 12 months	6,036	71,059
12 months and greater	55,681	60,049
Total Fair Value	61,717	131,108
Cost or amortized Cost
Less than 12 months	6,052	71,534
12 months and greater	56,975	60,827
Total Cost or Amortized Cost	63,027	132,361
Unrealized Loss
Less than 12 months	16	475
12 months and greater	1,294	778
Total Unrealized Loss	$ 1,310	$ 1,253