INVESTMENTS - Unrealized Losses (Details) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Investment positions with unrealized losses
Unrealized losses relative to total invested assets (as a percent)		1.40%
Debt securities
Fair value
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		$ 870,060	$ 372,467
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		216,531	254,270
Debt Securities, Available-for-sale, Unrealized Loss Position		1,086,591	626,737
Cost or amortized Cost
Less than 12 months		890,764	375,790
12 months and greater		225,695	259,335
Total Cost or Amortized Cost		1,116,459	635,125
Unrealized Loss
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		20,704	3,323
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		9,164	5,065
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss		$ 29,868	8,388
Percent to Total		100.00%
Debt securities | NAIC Rating 1 | AAA/AA/A | Aaa/Aa/A
Fair value
Debt Securities, Available-for-sale, Unrealized Loss Position		$ 908,318
Cost or amortized Cost
Total Cost or Amortized Cost		932,315
Unrealized Loss
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss		$ 23,997
Percent to Total		80.30%
Debt securities | NAIC Rating 2 | BBB | Baa
Fair value
Debt Securities, Available-for-sale, Unrealized Loss Position		$ 119,785
Cost or amortized Cost
Total Cost or Amortized Cost		124,031
Unrealized Loss
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss		$ 4,246
Percent to Total		14.20%
Debt securities | NAIC Rating 3 | BB | Ba
Fair value
Debt Securities, Available-for-sale, Unrealized Loss Position		$ 35,400
Cost or amortized Cost
Total Cost or Amortized Cost		36,317
Unrealized Loss
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss		$ 917
Percent to Total		3.10%
Debt securities | NAIC Rating 4 | B | B
Fair value
Debt Securities, Available-for-sale, Unrealized Loss Position		$ 21,267
Cost or amortized Cost
Total Cost or Amortized Cost		21,826
Unrealized Loss
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss		$ 559
Percent to Total		1.90%
Debt securities | NAIC Rating 5 | CCC | Caa
Fair value
Debt Securities, Available-for-sale, Unrealized Loss Position		$ 1,243
Cost or amortized Cost
Total Cost or Amortized Cost		1,354
Unrealized Loss
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss		$ 111
Percent to Total		0.40%
Debt securities | NAIC Six Rating Member | Standard Poors CC Or Lower Member | Ca
Fair value
Debt Securities, Available-for-sale, Unrealized Loss Position		$ 578
Cost or amortized Cost
Total Cost or Amortized Cost		616
Unrealized Loss
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss		$ 38
Percent to Total		0.10%
U.S. government
Fair value
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		$ 104,958	58,009
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		33,589	30,888
Debt Securities, Available-for-sale, Unrealized Loss Position		138,547	88,897
Cost or amortized Cost
Less than 12 months		106,718	58,443
12 months and greater		34,206	31,349
Total Cost or Amortized Cost		140,924	89,792
Unrealized Loss
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		1,760	434
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		617	461
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss		2,377	895
U.S. Agency
Fair value
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		15,422	10,917
Debt Securities, Available-for-sale, Unrealized Loss Position		15,422	10,917
Cost or amortized Cost
Less than 12 months		16,029	11,027
Total Cost or Amortized Cost		16,029	11,027
Unrealized Loss
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		607	110
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss		607	110
Non-U.S. govt. & agency
Fair value
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		6,208
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		1,687	1,840
Debt Securities, Available-for-sale, Unrealized Loss Position		7,895	1,840
Cost or amortized Cost
Less than 12 months		6,289
12 months and greater		1,897	1,896
Total Cost or Amortized Cost		8,186	1,896
Unrealized Loss
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		81
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		210	56
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss		291	56
Mortgage-backed
Fair value
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		199,344	122,130
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		106,745	111,306
Debt Securities, Available-for-sale, Unrealized Loss Position		306,089	233,436
Cost or amortized Cost
Less than 12 months		205,847	123,559
12 months and greater		111,880	113,955
Total Cost or Amortized Cost		317,727	237,514
Unrealized Loss
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		6,503	1,429
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		5,135	2,649
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss		11,638	4,078
ABS/CMBS
Fair value
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	[1]	66,925	23,406
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	[1]	18,028	21,587
Debt Securities, Available-for-sale, Unrealized Loss Position	[1]	84,953	44,993
Cost or amortized Cost
Less than 12 months	[1]	67,707	23,491
12 months and greater	[1]	18,279	21,817
Total Cost or Amortized Cost	[1]	85,986	45,308
Unrealized Loss
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	[1]	782	85
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	[1]	251	230
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	[1]	1,033	315
Corporate Debt
Fair value
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		391,379	86,946
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		34,255	28,600
Debt Securities, Available-for-sale, Unrealized Loss Position		425,634	115,546
Cost or amortized Cost
Less than 12 months		400,716	87,736
12 months and greater		36,452	29,491
Total Cost or Amortized Cost		437,168	117,227
Unrealized Loss
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		9,337	790
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		2,197	891
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss		11,534	1,681
Municipal
Fair value
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		85,824	71,059
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		22,227	60,049
Debt Securities, Available-for-sale, Unrealized Loss Position		108,051	131,108
Cost or amortized Cost
Less than 12 months		87,458	71,534
12 months and greater		22,981	60,827
Total Cost or Amortized Cost		110,439	132,361
Unrealized Loss
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		1,634	475
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		754	778
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss		$ 2,388	$ 1,253

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