INVESTMENTS - Unrealized Losses (Details) - USD ($) $ in Thousands		Jun. 30, 2019	Dec. 31, 2018
Investment positions with unrealized losses
Unrealized losses relative to total invested assets (as a percent)		0.20%
Unrealized Loss
Total Unrealized Loss		$ (3,600)
Debt securities
Fair value
Less than 12 months		61,374	$ 397,157
12 months and greater		208,816	584,017
Total Fair Value		270,190	981,174
Cost or amortized Cost
Less than 12 months		62,402	406,953
12 months and greater		211,422	603,710
Total Cost or Amortized Cost		273,824	1,010,663
Unrealized Loss
Less than 12 months		(1,028)	(9,796)
12 months and greater		(2,606)	(19,693)
Total Unrealized Loss		$ (3,634)	(29,489)
Percent to Total		100.00%
Debt securities | NAIC Rating 1 | AAA/AA/A | Aaa/Aa/A
Fair value
Total Fair Value		$ 201,403
Cost or amortized Cost
Total Cost or Amortized Cost		203,150
Unrealized Loss
Total Unrealized Loss		$ (1,747)
Percent to Total		48.10%
Debt securities | NAIC Rating 2 | BBB | Baa
Fair value
Total Fair Value		$ 21,213
Cost or amortized Cost
Total Cost or Amortized Cost		21,914
Unrealized Loss
Total Unrealized Loss		$ (701)
Percent to Total		19.30%
Debt securities | NAIC Rating 3 | BB | Ba
Fair value
Total Fair Value		$ 21,950
Cost or amortized Cost
Total Cost or Amortized Cost		22,336
Unrealized Loss
Total Unrealized Loss		$ (386)
Percent to Total		10.60%
Debt securities | NAIC Rating 4 | B | B
Fair value
Total Fair Value		$ 24,148
Cost or amortized Cost
Total Cost or Amortized Cost		24,877
Unrealized Loss
Total Unrealized Loss		$ (729)
Percent to Total		20.10%
Debt securities | NAIC Rating 5 | CCC | Caa
Fair value
Total Fair Value		$ 1,476
Cost or amortized Cost
Total Cost or Amortized Cost		1,547
Unrealized Loss
Total Unrealized Loss		$ (71)
Percent to Total		2.00%
U.S. government
Fair value
Less than 12 months			7,249
12 months and greater		$ 23,525	76,073
Total Fair Value		23,525	83,322
Cost or amortized Cost
Less than 12 months			7,270
12 months and greater		23,626	77,037
Total Cost or Amortized Cost		23,626	84,307
Unrealized Loss
Less than 12 months			(21)
12 months and greater		(101)	(964)
Total Unrealized Loss		(101)	(985)
U.S. Agency
Fair value
12 months and greater			8,843
Total Fair Value			8,843
Cost or amortized Cost
12 months and greater			9,058
Total Cost or Amortized Cost			9,058
Unrealized Loss
12 months and greater			(215)
Total Unrealized Loss			(215)
Non-U.S. govt. & agency
Fair value
Less than 12 months			5,432
12 months and greater		3,625	2,207
Total Fair Value		3,625	7,639
Cost or amortized Cost
Less than 12 months			5,571
12 months and greater		3,954	2,599
Total Cost or Amortized Cost		3,954	8,170
Unrealized Loss
Less than 12 months			(139)
12 months and greater		(329)	(392)
Total Unrealized Loss		(329)	(531)
Mortgage-backed
Fair value
Less than 12 months		1	25,345
12 months and greater		129,203	261,325
Total Fair Value		129,204	286,670
Cost or amortized Cost
Less than 12 months		1	25,486
12 months and greater		130,603	270,632
Total Cost or Amortized Cost		130,604	296,118
Unrealized Loss
Less than 12 months			(141)
12 months and greater		(1,400)	(9,307)
Total Unrealized Loss		(1,400)	(9,448)
ABS/CMBS
Fair value
Less than 12 months	[1]	16,755	46,918
12 months and greater	[1]	11,628	32,137
Total Fair Value	[1]	28,383	79,055
Cost or amortized Cost
Less than 12 months	[1]	16,823	47,146
12 months and greater	[1]	11,707	32,785
Total Cost or Amortized Cost	[1]	28,530	79,931
Unrealized Loss
Less than 12 months	[1]	(68)	(228)
12 months and greater	[1]	(79)	(648)
Total Unrealized Loss	[1]	(147)	(876)
Corporate Debt
Fair value
Less than 12 months		44,618	306,177
12 months and greater		32,689	147,751
Total Fair Value		77,307	453,928
Cost or amortized Cost
Less than 12 months		45,578	315,428
12 months and greater		33,361	154,624
Total Cost or Amortized Cost		78,939	470,052
Unrealized Loss
Less than 12 months		(960)	(9,251)
12 months and greater		(672)	(6,873)
Total Unrealized Loss		(1,632)	(16,124)
Municipal
Fair value
Less than 12 months			6,036
12 months and greater		8,146	55,681
Total Fair Value		8,146	61,717
Cost or amortized Cost
Less than 12 months			6,052
12 months and greater		8,171	56,975
Total Cost or Amortized Cost		8,171	63,027
Unrealized Loss
Less than 12 months			(16)
12 months and greater		(25)	(1,294)
Total Unrealized Loss		$ (25)	$ (1,310)

[1] Non-agency asset-backed and commercial mortgage-backed