INVESTMENTS - Types of Debt and Equity Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt securities
Fair value
Less than 12 months	$ 169,311	$ 397,157
12 months and greater	77,565	584,017
Total Fair Value	246,876	981,174
Cost or amortized Cost
Amortized cost, less than 12 months	171,028	406,953
12 months and greater	78,501	603,710
Total Cost or Amortized Cost	249,529	1,010,663
Unrealized Loss
Less than 12 months	1,717	9,796
12 months and greater	936	19,693
Total Unrealized Loss	2,653	29,489
U.S. government
Fair value
Less than 12 months	2,505	7,249
12 months and greater	8,463	76,073
Total Fair Value	10,968	83,322
Cost or amortized Cost
Amortized cost, less than 12 months	2,506	7,270
12 months and greater	8,494	77,037
Total Cost or Amortized Cost	11,000	84,307
Unrealized Loss
Less than 12 months	1	21
12 months and greater	31	964
Total Unrealized Loss	32	985
U.S. Agency
Fair value
Less than 12 months	6,794
12 months and greater		8,843
Total Fair Value	6,794	8,843
Cost or amortized Cost
Amortized cost, less than 12 months	6,836
12 months and greater		9,058
Total Cost or Amortized Cost	6,836	9,058
Unrealized Loss
Less than 12 months	42
12 months and greater		215
Total Unrealized Loss	42	215
Non-U.S. govt. & agency
Fair value
Less than 12 months		5,432
12 months and greater		2,207
Total Fair Value		7,639
Cost or amortized Cost
Amortized cost, less than 12 months		5,571
12 months and greater		2,599
Total Cost or Amortized Cost		8,170
Unrealized Loss
Less than 12 months		139
12 months and greater		392
Total Unrealized Loss		531
Agency MBS
Fair value
Less than 12 months	21,548	25,345
12 months and greater	41,718	261,325
Total Fair Value	63,266	286,670
Cost or amortized Cost
Amortized cost, less than 12 months	21,664	25,486
12 months and greater	42,165	270,632
Total Cost or Amortized Cost	63,829	296,118
Unrealized Loss
Less than 12 months	116	141
12 months and greater	447	9,307
Total Unrealized Loss	563	9,448
ABS/CMBS/MBS
Fair value
Less than 12 months	74,968	46,918
12 months and greater	18,036	32,137
Total Fair Value	93,004	79,055
Cost or amortized Cost
Amortized cost, less than 12 months	75,332	47,146
12 months and greater	18,148	32,785
Total Cost or Amortized Cost	93,480	79,931
Unrealized Loss
Less than 12 months	364	228
12 months and greater	112	648
Total Unrealized Loss	476	876
Corporate Debt
Fair value
Less than 12 months	16,478	306,177
12 months and greater	9,348	147,751
Total Fair Value	25,826	453,928
Cost or amortized Cost
Amortized cost, less than 12 months	16,950	315,428
12 months and greater	9,694	154,624
Total Cost or Amortized Cost	26,644	470,052
Unrealized Loss
Less than 12 months	472	9,251
12 months and greater	346	6,873
Total Unrealized Loss	818	16,124
Municipal
Fair value
Less than 12 months	47,018	6,036
12 months and greater		55,681
Total Fair Value	47,018	61,717
Cost or amortized Cost
Amortized cost, less than 12 months	47,740	6,052
12 months and greater		56,975
Total Cost or Amortized Cost	47,740	63,027
Unrealized Loss
Less than 12 months	722	16
12 months and greater		1,294
Total Unrealized Loss	$ 722	$ 1,310