INVESTMENTS - Unrealized Losses (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Investment positions with unrealized losses
Unrealized losses relative to total invested assets (as a percent)	0.10%
Unrealized Loss
Total Unrealized Loss	$ (2,500)
Debt securities
Fair value
Less than 12 months	74,891	$ 397,157
12 months and greater	91,106	584,017
Total Fair Value	165,997	981,174
Cost or amortized Cost
Less than 12 months	75,760	406,953
12 months and greater	92,737	603,710
Total Cost or Amortized Cost	168,497	1,010,663
Unrealized Loss
Less than 12 months	(869)	(9,796)
12 months and greater	(1,631)	(19,693)
Total Unrealized Loss	$ (2,500)	(29,489)
Percent to Total	100.00%
Debt securities | NAIC Rating 1 | AAA/AA/A | Aaa/Aa/A
Fair value
Total Fair Value	$ 121,660
Cost or amortized Cost
Total Cost or Amortized Cost	122,495
Unrealized Loss
Total Unrealized Loss	$ (835)
Percent to Total	33.40%
Debt securities | NAIC Rating 2 | BBB | Baa
Fair value
Total Fair Value	$ 11,683
Cost or amortized Cost
Total Cost or Amortized Cost	12,031
Unrealized Loss
Total Unrealized Loss	$ (348)
Percent to Total	13.90%
Debt securities | NAIC Rating 3 | BB | Ba
Fair value
Total Fair Value	$ 11,981
Cost or amortized Cost
Total Cost or Amortized Cost	12,481
Unrealized Loss
Total Unrealized Loss	$ (500)
Percent to Total	20.00%
Debt securities | NAIC Rating 4 | B | B
Fair value
Total Fair Value	$ 20,245
Cost or amortized Cost
Total Cost or Amortized Cost	20,977
Unrealized Loss
Total Unrealized Loss	$ (732)
Percent to Total	29.30%
Debt securities | NAIC Rating 5 | CCC | Caa
Fair value
Total Fair Value	$ 428
Cost or amortized Cost
Total Cost or Amortized Cost	513
Unrealized Loss
Total Unrealized Loss	$ (85)
Percent to Total	3.40%
U.S. government
Fair value
Less than 12 months	$ 998	7,249
12 months and greater	15,717	76,073
Total Fair Value	16,715	83,322
Cost or amortized Cost
Less than 12 months	999	7,270
12 months and greater	15,777	77,037
Total Cost or Amortized Cost	16,776	84,307
Unrealized Loss
Less than 12 months	(1)	(21)
12 months and greater	(60)	(964)
Total Unrealized Loss	(61)	(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	1,710	2,207
Total Fair Value	1,710	7,639
Cost or amortized Cost
Less than 12 months		5,571
12 months and greater	1,899	2,599
Total Cost or Amortized Cost	1,899	8,170
Unrealized Loss
Less than 12 months		(139)
12 months and greater	(189)	(392)
Total Unrealized Loss	(189)	(531)
Mortgage-backed
Fair value
Less than 12 months	6,209	25,345
12 months and greater	49,352	261,325
Total Fair Value	55,561	286,670
Cost or amortized Cost
Less than 12 months	6,215	25,486
12 months and greater	49,917	270,632
Total Cost or Amortized Cost	56,132	296,118
Unrealized Loss
Less than 12 months	(6)	(141)
12 months and greater	(565)	(9,307)
Total Unrealized Loss	(571)	(9,448)
ABS/CMBS
Fair value
Less than 12 months	26,521	46,918
12 months and greater	8,101	32,137
Total Fair Value	34,622	79,055
Cost or amortized Cost
Less than 12 months	26,600	47,146
12 months and greater	8,166	32,785
Total Cost or Amortized Cost	34,766	79,931
Unrealized Loss
Less than 12 months	(79)	(228)
12 months and greater	(65)	(648)
Total Unrealized Loss	(144)	(876)
Corporate Debt
Fair value
Less than 12 months	32,584	306,177
12 months and greater	16,226	147,751
Total Fair Value	48,810	453,928
Cost or amortized Cost
Less than 12 months	33,341	315,428
12 months and greater	16,978	154,624
Total Cost or Amortized Cost	50,319	470,052
Unrealized Loss
Less than 12 months	(757)	(9,251)
12 months and greater	(752)	(6,873)
Total Unrealized Loss	(1,509)	(16,124)
Municipal
Fair value
Less than 12 months	8,579	6,036
12 months and greater		55,681
Total Fair Value	8,579	61,717
Cost or amortized Cost
Less than 12 months	8,605	6,052
12 months and greater		56,975
Total Cost or Amortized Cost	8,605	63,027
Unrealized Loss
Less than 12 months	(26)	(16)
12 months and greater		(1,294)
Total Unrealized Loss	$ (26)	$ (1,310)