INVESTMENTS - Unrealized Losses (Details) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Investment positions with unrealized losses
Unrealized losses relative to total invested assets (as a percent)		1.60%
Debt securities
Fair value
Less than 12 months		$ 775,537	$ 372,467
12 months and greater		474,878	254,270
Total Fair Value		1,250,415	626,737
Cost or amortized Cost
Less than 12 months		789,058	375,790
12 months and greater		496,508	259,335
Total Cost or Amortized Cost		1,285,566	635,125
Unrealized Loss
Less than 12 months		13,521	3,323
12 months and greater		21,630	5,065
Total Unrealized Loss		$ 35,151	8,388
Percent to Total		100.00%
Debt securities | NAIC Rating 1 | AAA/AA/A | Aaa/Aa/A
Fair value
Total Fair Value		$ 1,074,126
Cost or amortized Cost
Total Cost or Amortized Cost		1,103,857
Unrealized Loss
Total Unrealized Loss		$ 29,731
Percent to Total		84.60%
Debt securities | NAIC Rating 2 | BBB | Baa
Fair value
Total Fair Value		$ 131,948
Cost or amortized Cost
Total Cost or Amortized Cost		136,227
Unrealized Loss
Total Unrealized Loss		$ 4,279
Percent to Total		12.20%
Debt securities | NAIC Rating 3 | BB | Ba
Fair value
Total Fair Value		$ 28,491
Cost or amortized Cost
Total Cost or Amortized Cost		29,221
Unrealized Loss
Total Unrealized Loss		$ 730
Percent to Total		2.10%
Debt securities | NAIC Rating 4 | B | B
Fair value
Total Fair Value		$ 14,386
Cost or amortized Cost
Total Cost or Amortized Cost		14,712
Unrealized Loss
Total Unrealized Loss		$ 326
Percent to Total		0.90%
Debt securities | NAIC Rating 5 | CCC | Caa
Fair value
Total Fair Value		$ 1,464
Cost or amortized Cost
Total Cost or Amortized Cost		1,549
Unrealized Loss
Total Unrealized Loss		$ 85
Percent to Total		0.20%
U.S. government
Fair value
Less than 12 months		$ 72,347	58,009
12 months and greater		99,558	30,888
Total Fair Value		171,905	88,897
Cost or amortized Cost
Less than 12 months		72,981	58,443
12 months and greater		102,484	31,349
Total Cost or Amortized Cost		175,465	89,792
Unrealized Loss
Less than 12 months		634	434
12 months and greater		2,926	461
Total Unrealized Loss		3,560	895
U.S. Agency
Fair value
Less than 12 months		17,408	10,917
12 months and greater		10,384
Total Fair Value		27,792	10,917
Cost or amortized Cost
Less than 12 months		17,622	11,027
12 months and greater		11,025
Total Cost or Amortized Cost		28,647	11,027
Unrealized Loss
Less than 12 months		214	110
12 months and greater		641
Total Unrealized Loss		855	110
Non-U.S. govt. & agency
Fair value
Less than 12 months		6,214
12 months and greater		1,693	1,840
Total Fair Value		7,907	1,840
Cost or amortized Cost
Less than 12 months		6,280
12 months and greater		1,898	1,896
Total Cost or Amortized Cost		8,178	1,896
Unrealized Loss
Less than 12 months		66
12 months and greater		205	56
Total Unrealized Loss		271	56
Mortgage-backed
Fair value
Less than 12 months		152,341	122,130
12 months and greater		206,541	111,306
Total Fair Value		358,882	233,436
Cost or amortized Cost
Less than 12 months		155,615	123,559
12 months and greater		218,016	113,955
Total Cost or Amortized Cost		373,631	237,514
Unrealized Loss
Less than 12 months		3,274	1,429
12 months and greater		11,475	2,649
Total Unrealized Loss		14,749	4,078
ABS/CMBS
Fair value
Less than 12 months	[1]	82,967	23,406
12 months and greater	[1]	28,580	21,587
Total Fair Value	[1]	111,547	44,993
Cost or amortized Cost
Less than 12 months	[1]	83,647	23,491
12 months and greater	[1]	29,183	21,817
Total Cost or Amortized Cost	[1]	112,830	45,308
Unrealized Loss
Less than 12 months	[1]	680	85
12 months and greater	[1]	603	230
Total Unrealized Loss	[1]	1,283	315
Corporate Debt
Fair value
Less than 12 months		387,421	86,946
12 months and greater		84,835	28,600
Total Fair Value		472,256	115,546
Cost or amortized Cost
Less than 12 months		395,327	87,736
12 months and greater		88,830	29,491
Total Cost or Amortized Cost		484,157	117,227
Unrealized Loss
Less than 12 months		7,906	790
12 months and greater		3,995	891
Total Unrealized Loss		11,901	1,681
Municipal
Fair value
Less than 12 months		56,839	71,059
12 months and greater		43,287	60,049
Total Fair Value		100,126	131,108
Cost or amortized Cost
Less than 12 months		57,586	71,534
12 months and greater		45,072	60,827
Total Cost or Amortized Cost		102,658	132,361
Unrealized Loss
Less than 12 months		747	475
12 months and greater		1,785	778
Total Unrealized Loss		$ 2,532	$ 1,253

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