INVESTMENTS - Types of Debt and Equity Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair value
Less than 12 months	$ 683,633	$ 500,405
12 Months and Greater	28,517	44,476
Total Fair value	712,150	544,881
Cost or amortized Cost
Less than 12 months	698,230	514,250
12 months and greater	32,516	49,883
Total Cost or Amortized Cost	730,746	564,133
Unrealized Loss
Less than 12 months	(14,597)	(13,845)
12 Months and Greater	(3,999)	(5,407)
Total Unrealized Loss	(18,596)	(19,252)
Debt securities
Fair value
Less than 12 months	676,195	483,929
12 Months and Greater	26,544	44,476
Total Fair value	702,739	528,405
Cost or amortized Cost
Less than 12 months	690,201	496,318
12 months and greater	29,868	49,883
Total Cost or Amortized Cost	720,069	546,201
Unrealized Loss
Less than 12 months	(14,006)	(12,389)
12 Months and Greater	(3,324)	(5,407)
Total Unrealized Loss	(17,330)	(17,796)
U.S. government
Fair value
Less than 12 months	48,500	36,000
Total Fair value	48,500	36,000
Cost or amortized Cost
Less than 12 months	49,079	36,112
Total Cost or Amortized Cost	49,079	36,112
Unrealized Loss
Less than 12 months	(579)	(112)
Total Unrealized Loss	(579)	(112)
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	7,647	4,478
Total Fair value	7,647	4,478
Cost or amortized Cost
Less than 12 months	8,015	5,035
Total Cost or Amortized Cost	8,015	5,035
Unrealized Loss
Less than 12 months	(368)	(557)
Total Unrealized Loss	(368)	(557)
Mortgage-backed
Fair value
Less than 12 months	175,858	100,424
12 Months and Greater	5,737	18,520
Total Fair value	181,595	118,944
Cost or amortized Cost
Less than 12 months	179,238	101,473
12 months and greater	5,925	19,090
Total Cost or Amortized Cost	185,163	120,563
Unrealized Loss
Less than 12 months	(3,380)	(1,049)
12 Months and Greater	(188)	(570)
Total Unrealized Loss	(3,568)	(1,619)
ABS/CMBS
Fair value
Less than 12 months	48,907	51,091
12 Months and Greater	5,272	8,364
Total Fair value	54,179	59,455
Cost or amortized Cost
Less than 12 months	49,372	51,562
12 months and greater	5,364	8,499
Total Cost or Amortized Cost	54,736	60,061
Unrealized Loss
Less than 12 months	(465)	(471)
12 Months and Greater	(92)	(135)
Total Unrealized Loss	(557)	(606)
Corporate Debt
Fair value
Less than 12 months	144,353	275,404
12 Months and Greater	15,535	15,174
Total Fair value	159,888	290,578
Cost or amortized Cost
Less than 12 months	146,979	285,515
12 months and greater	18,579	19,870
Total Cost or Amortized Cost	165,558	305,385
Unrealized Loss
Less than 12 months	(2,626)	(10,111)
12 Months and Greater	(3,044)	(4,696)
Total Unrealized Loss	(5,670)	(14,807)
Municipal
Fair value
Less than 12 months	250,930	8,462
12 Months and Greater		2,418
Total Fair value	250,930	10,880
Cost or amortized Cost
Less than 12 months	257,518	8,504
12 months and greater		2,424
Total Cost or Amortized Cost	257,518	10,928
Unrealized Loss
Less than 12 months	(6,588)	(42)
12 Months and Greater		(6)
Total Unrealized Loss	(6,588)	(48)
Equity securities
Fair value
Less than 12 months	7,438	16,476
12 Months and Greater	1,973
Total Fair value	9,411	16,476
Cost or amortized Cost
Less than 12 months	8,029	17,932
12 months and greater	2,648
Total Cost or Amortized Cost	10,677	17,932
Unrealized Loss
Less than 12 months	(591)	(1,456)
12 Months and Greater	(675)
Total Unrealized Loss	$ (1,266)	$ (1,456)