INVESTMENTS - Unrealized Losses (Details) - USD ($) $ in Thousands		Mar. 31, 2019	Dec. 31, 2018
Investment positions with unrealized losses
Unrealized losses relative to total invested assets (as a percent)		0.50%
Debt securities
Fair value
Less than 12 months		$ 92,893	$ 397,157
12 months and greater		463,648	584,017
Total Fair Value		556,541	981,174
Cost or amortized Cost
Less than 12 months		94,606	406,953
12 months and greater		472,562	603,710
Total Cost or Amortized Cost		567,168	1,010,663
Unrealized Loss
Less than 12 months		(1,713)	(9,796)
12 months and greater		(8,914)	(19,693)
Total Unrealized Loss		$ (10,627)	(29,489)
Percent to Total		100.00%
Debt securities | NAIC Rating 1 | AAA/AA/A | Aaa/Aa/A
Fair value
Total Fair Value		$ 427,442
Cost or amortized Cost
Total Cost or Amortized Cost		434,824
Unrealized Loss
Total Unrealized Loss		$ (7,382)
Percent to Total		69.50%
Debt securities | NAIC Rating 2 | BBB | Baa
Fair value
Total Fair Value		$ 57,548
Cost or amortized Cost
Total Cost or Amortized Cost		59,320
Unrealized Loss
Total Unrealized Loss		$ (1,772)
Percent to Total		16.70%
Debt securities | NAIC Rating 3 | BB | Ba
Fair value
Total Fair Value		$ 34,375
Cost or amortized Cost
Total Cost or Amortized Cost		34,925
Unrealized Loss
Total Unrealized Loss		$ (550)
Percent to Total		5.20%
Debt securities | NAIC Rating 4 | B | B
Fair value
Total Fair Value		$ 35,408
Cost or amortized Cost
Total Cost or Amortized Cost		36,178
Unrealized Loss
Total Unrealized Loss		$ (770)
Percent to Total		7.20%
Debt securities | NAIC Rating 5 | CCC | Caa
Fair value
Total Fair Value		$ 1,768
Cost or amortized Cost
Total Cost or Amortized Cost		1,921
Unrealized Loss
Total Unrealized Loss		$ (153)
Percent to Total		1.40%
U.S. government
Fair value
Less than 12 months			7,249
12 months and greater		$ 46,641	76,073
Total Fair Value		46,641	83,322
Cost or amortized Cost
Less than 12 months			7,270
12 months and greater		47,025	77,037
Total Cost or Amortized Cost		47,025	84,307
Unrealized Loss
Less than 12 months			(21)
12 months and greater		(384)	(964)
Total Unrealized Loss		(384)	(985)
U.S. Agency
Fair value
12 months and greater		3,988	8,843
Total Fair Value		3,988	8,843
Cost or amortized Cost
12 months and greater		4,005	9,058
Total Cost or Amortized Cost		4,005	9,058
Unrealized Loss
12 months and greater		(17)	(215)
Total Unrealized Loss		(17)	(215)
Non-U.S. govt. & agency
Fair value
Less than 12 months			5,432
12 months and greater		3,669	2,207
Total Fair Value		3,669	7,639
Cost or amortized Cost
Less than 12 months			5,571
12 months and greater		3,962	2,599
Total Cost or Amortized Cost		3,962	8,170
Unrealized Loss
Less than 12 months			(139)
12 months and greater		(293)	(392)
Total Unrealized Loss		(293)	(531)
Mortgage-backed
Fair value
Less than 12 months		1,388	25,345
12 months and greater		239,998	261,325
Total Fair Value		241,386	286,670
Cost or amortized Cost
Less than 12 months		1,389	25,486
12 months and greater		245,490	270,632
Total Cost or Amortized Cost		246,879	296,118
Unrealized Loss
Less than 12 months		(1)	(141)
12 months and greater		(5,492)	(9,307)
Total Unrealized Loss		(5,493)	(9,448)
ABS/CMBS
Fair value
Less than 12 months	[1]	20,570	46,918
12 months and greater	[1]	30,341	32,137
Total Fair Value	[1]	50,911	79,055
Cost or amortized Cost
Less than 12 months	[1]	20,689	47,146
12 months and greater	[1]	30,563	32,785
Total Cost or Amortized Cost	[1]	51,252	79,931
Unrealized Loss
Less than 12 months	[1]	(119)	(228)
12 months and greater	[1]	(222)	(648)
Total Unrealized Loss	[1]	(341)	(876)
Corporate Debt
Fair value
Less than 12 months		70,935	306,177
12 months and greater		120,782	147,751
Total Fair Value		191,717	453,928
Cost or amortized Cost
Less than 12 months		72,528	315,428
12 months and greater		123,099	154,624
Total Cost or Amortized Cost		195,627	470,052
Unrealized Loss
Less than 12 months		(1,593)	(9,251)
12 months and greater		(2,317)	(6,873)
Total Unrealized Loss		(3,910)	(16,124)
Municipal
Fair value
Less than 12 months			6,036
12 months and greater		18,229	55,681
Total Fair Value		18,229	61,717
Cost or amortized Cost
Less than 12 months			6,052
12 months and greater		18,418	56,975
Total Cost or Amortized Cost		18,418	63,027
Unrealized Loss
Less than 12 months			(16)
12 months and greater		(189)	(1,294)
Total Unrealized Loss		$ (189)	$ (1,310)

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