INVESTMENTS - Types of Debt and Equity Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Fair value
Less than 12 months	$ 500,405	$ 227,874
12 Months and Greater	44,476	106,657
Total Fair value	544,881	334,531
Cost or amortized Cost
Less than 12 months	514,250	233,338
12 months and greater	49,883	108,312
Total Cost or Amortized Cost	564,133	341,650
Unrealized Loss
Less than 12 months	(13,845)	(5,464)
12 Months and Greater	(5,407)	(1,655)
Total Unrealized Loss	(19,252)	(7,119)
Debt securities
Fair value
Less than 12 months	483,929	217,037
12 Months and Greater	44,476	106,657
Total Fair value	528,405	323,694
Cost or amortized Cost
Less than 12 months	496,318	221,503
12 months and greater	49,883	108,312
Total Cost or Amortized Cost	546,201	329,815
Unrealized Loss
Less than 12 months	(12,389)	(4,466)
12 Months and Greater	(5,407)	(1,655)
Total Unrealized Loss	(17,796)	(6,121)
U.S. government
Fair value
Less than 12 months	36,000	4,416
Total Fair value	36,000	4,416
Cost or amortized Cost
Less than 12 months	36,112	4,427
Total Cost or Amortized Cost	36,112	4,427
Unrealized Loss
Less than 12 months	(112)	(11)
Total Unrealized Loss	(112)	(11)
U.S. Agency
Fair value
Less than 12 months	8,070
Total Fair value	8,070
Cost or amortized Cost
Less than 12 months	8,117
Total Cost or Amortized Cost	8,117
Unrealized Loss
Less than 12 months	(47)
Total Unrealized Loss	(47)
Non-U.S. govt. & agency
Fair value
Less than 12 months	4,478
Total Fair value	4,478
Cost or amortized Cost
Less than 12 months	5,035
Total Cost or Amortized Cost	5,035
Unrealized Loss
Less than 12 months	(557)
Total Unrealized Loss	(557)
Agency MBS
Fair value
Less than 12 months	100,424	12,840
12 Months and Greater	18,520	61,534
Total Fair value	118,944	74,374
Cost or amortized Cost
Less than 12 months	101,473	12,947
12 months and greater	19,090	62,803
Total Cost or Amortized Cost	120,563	75,750
Unrealized Loss
Less than 12 months	(1,049)	(107)
12 Months and Greater	(570)	(1,269)
Total Unrealized Loss	(1,619)	(1,376)
ABS/CMBS
Fair value
Less than 12 months	51,091	63,782
12 Months and Greater	8,364	11,616
Total Fair value	59,455	75,398
Cost or amortized Cost
Less than 12 months	51,562	64,084
12 months and greater	8,499	11,725
Total Cost or Amortized Cost	60,061	75,809
Unrealized Loss
Less than 12 months	(471)	(302)
12 Months and Greater	(135)	(109)
Total Unrealized Loss	(606)	(411)
Corporate
Fair value
Less than 12 months	275,404	123,617
12 Months and Greater	15,174	14,488
Total Fair value	290,578	138,105
Cost or amortized Cost
Less than 12 months	285,515	127,634
12 months and greater	19,870	14,661
Total Cost or Amortized Cost	305,385	142,295
Unrealized Loss
Less than 12 months	(10,111)	(4,017)
12 Months and Greater	(4,696)	(173)
Total Unrealized Loss	(14,807)	(4,190)
Municipal
Fair value
Less than 12 months	8,462	12,382
12 Months and Greater	2,418	19,019
Total Fair value	10,880	31,401
Cost or amortized Cost
Less than 12 months	8,504	12,411
12 months and greater	2,424	19,123
Total Cost or Amortized Cost	10,928	31,534
Unrealized Loss
Less than 12 months	(42)	(29)
12 Months and Greater	(6)	(104)
Total Unrealized Loss	(48)	(133)
Equity securities
Fair value
Less than 12 months	16,476	10,837
Total Fair value	16,476	10,837
Cost or amortized Cost
Less than 12 months	17,932	11,835
Total Cost or Amortized Cost	17,932	11,835
Unrealized Loss
Less than 12 months	(1,456)	(998)
Total Unrealized Loss	$ (1,456)	$ (998)