INVESTMENTS - Unrealized Losses (Details) - USD ($) $ in Thousands		Sep. 30, 2017	Dec. 31, 2016
Investment positions with unrealized losses
Unrealized losses relative to total invested assets (as a percent)		0.40%
Fair value
Less than 12 months		$ 373,323	$ 683,633
12 Months and Greater		159,973	28,517
Total Fair value		533,296	712,150
Cost or amortized Cost
Less than 12 months		377,606	698,230
12 months and greater		164,443	32,516
Total Cost or Amortized Cost		542,049	730,746
Unrealized Loss
Less than 12 months		(4,283)	(14,597)
12 Months and Greater		(4,470)	(3,999)
Total Unrealized Loss		(8,753)	(18,596)
Debt securities
Fair value
Less than 12 months		364,627	676,195
12 Months and Greater		155,467	26,544
Total Fair value		520,094	702,739
Cost or amortized Cost
Less than 12 months		367,890	690,201
12 months and greater		159,265	29,868
Total Cost or Amortized Cost		527,155	720,069
Unrealized Loss
Less than 12 months		(3,263)	(14,006)
12 Months and Greater		(3,798)	(3,324)
Total Unrealized Loss		$ (7,061)	(17,330)
Percent to Total		100.00%
Debt securities | NAIC Rating 1 | AAA/AA/A | Aaa/Aa/A
Fair value
Total Fair value		$ 478,024
Cost or amortized Cost
Total Cost or Amortized Cost		483,555
Unrealized Loss
Total Unrealized Loss		$ (5,531)
Percent to Total		78.30%
Debt securities | NAIC Rating 2 | BBB | Baa
Fair value
Total Fair value		$ 27,669
Cost or amortized Cost
Total Cost or Amortized Cost		27,896
Unrealized Loss
Total Unrealized Loss		$ (227)
Percent to Total		3.20%
Debt securities | NAIC Rating 3 | BB | Ba
Fair value
Total Fair value		$ 5,639
Cost or amortized Cost
Total Cost or Amortized Cost		5,693
Unrealized Loss
Total Unrealized Loss		$ (54)
Percent to Total		0.80%
Debt securities | NAIC Rating 4 | B
Fair value
Total Fair value		$ 7,171
Cost or amortized Cost
Total Cost or Amortized Cost		7,945
Unrealized Loss
Total Unrealized Loss		$ (774)
Percent to Total		11.00%
Debt securities | NAIC Rating 5 | CCC or lower
Fair value
Total Fair value		$ 1,591
Cost or amortized Cost
Total Cost or Amortized Cost		2,066
Unrealized Loss
Total Unrealized Loss		$ (475)
Percent to Total		6.70%
U.S. government
Fair value
Less than 12 months		$ 54,686	48,500
12 Months and Greater		3,008
Total Fair value		57,694	48,500
Cost or amortized Cost
Less than 12 months		55,028	49,079
12 months and greater		3,040
Total Cost or Amortized Cost		58,068	49,079
Unrealized Loss
Less than 12 months		(342)	(579)
12 Months and Greater		(32)
Total Unrealized Loss		(374)	(579)
U.S. Agency
Fair value
Less than 12 months		7,921
Total Fair value		7,921
Cost or amortized Cost
Less than 12 months		8,007
Total Cost or Amortized Cost		8,007
Unrealized Loss
Less than 12 months		(86)
Total Unrealized Loss		(86)
Non-U.S. govt. & agency
Fair value
Less than 12 months		1,973	7,647
12 Months and Greater		1,865
Total Fair value		3,838	7,647
Cost or amortized Cost
Less than 12 months		1,988	8,015
12 months and greater		1,896
Total Cost or Amortized Cost		3,884	8,015
Unrealized Loss
Less than 12 months		(15)	(368)
12 Months and Greater		(31)
Total Unrealized Loss		(46)	(368)
Mortgage-backed
Fair value
Less than 12 months		156,908	175,858
12 Months and Greater		56,025	5,737
Total Fair value		212,933	181,595
Cost or amortized Cost
Less than 12 months		158,567	179,238
12 months and greater		57,319	5,925
Total Cost or Amortized Cost		215,886	185,163
Unrealized Loss
Less than 12 months		(1,659)	(3,380)
12 Months and Greater		(1,294)	(188)
Total Unrealized Loss		(2,953)	(3,568)
ABS/CMBS
Fair value
Less than 12 months	[1]	22,971	48,907
12 Months and Greater	[1]	15,577	5,272
Total Fair value	[1]	38,548	54,179
Cost or amortized Cost
Less than 12 months	[1]	23,032	49,372
12 months and greater	[1]	15,732	5,364
Total Cost or Amortized Cost	[1]	38,764	54,736
Unrealized Loss
Less than 12 months	[1]	(61)	(465)
12 Months and Greater	[1]	(155)	(92)
Total Unrealized Loss	[1]	(216)	(557)
Corporate Debt
Fair value
Less than 12 months		58,328	144,353
12 Months and Greater		24,028	15,535
Total Fair value		82,356	159,888
Cost or amortized Cost
Less than 12 months		58,881	146,979
12 months and greater		25,509	18,579
Total Cost or Amortized Cost		84,390	165,558
Unrealized Loss
Less than 12 months		(553)	(2,626)
12 Months and Greater		(1,481)	(3,044)
Total Unrealized Loss		(2,034)	(5,670)
Municipal
Fair value
Less than 12 months		61,840	250,930
12 Months and Greater		54,964
Total Fair value		116,804	250,930
Cost or amortized Cost
Less than 12 months		62,387	257,518
12 months and greater		55,769
Total Cost or Amortized Cost		118,156	257,518
Unrealized Loss
Less than 12 months		(547)	(6,588)
12 Months and Greater		(805)
Total Unrealized Loss		(1,352)	(6,588)
Equity securities
Fair value
Less than 12 months		8,696	7,438
12 Months and Greater		4,506	1,973
Total Fair value		13,202	9,411
Cost or amortized Cost
Less than 12 months		9,716	8,029
12 months and greater		5,178	2,648
Total Cost or Amortized Cost		14,894	10,677
Unrealized Loss
Less than 12 months		(1,020)	(591)
12 Months and Greater		(672)	(675)
Total Unrealized Loss		$ (1,692)	$ (1,266)

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