INVESTMENTS - Unrealized Losses (Details) - USD ($) $ in Thousands		Mar. 31, 2017	Dec. 31, 2016
Investment positions with unrealized losses
Unrealized losses relative to total invested assets (as a percent)		0.70%
Fair value
Less than 12 months		$ 580,038	$ 683,633
12 Months and Greater		24,971	28,517
Total Fair value		605,009	712,150
Cost or amortized Cost
Less than 12 months		591,719	698,230
12 months and greater		27,219	32,516
Total Cost or Amortized Cost		618,938	730,746
Unrealized Loss
Less than 12 months		(11,681)	(14,597)
12 Months and Greater		(2,248)	(3,999)
Total Unrealized Loss		(13,929)	(18,596)
Debt securities
Fair value
Less than 12 months		570,486	676,195
12 Months and Greater		21,046	26,544
Total Fair value		591,532	702,739
Cost or amortized Cost
Less than 12 months		581,151	690,201
12 months and greater		22,575	29,868
Total Cost or Amortized Cost		603,726	720,069
Unrealized Loss
Less than 12 months		(10,665)	(14,006)
12 Months and Greater		(1,529)	(3,324)
Total Unrealized Loss		$ (12,194)	(17,330)
Percent to Total		100.00%
Debt securities | NAIC Rating 1 | AAA/AA/A | Aaa/Aa/A
Fair value
Total Fair value		$ 515,405
Cost or amortized Cost
Total Cost or Amortized Cost		525,282
Unrealized Loss
Total Unrealized Loss		$ (9,877)
Percent to Total		81.00%
Debt securities | NAIC Rating 2 | BBB | Baa
Fair value
Total Fair value		$ 62,204
Cost or amortized Cost
Total Cost or Amortized Cost		63,482
Unrealized Loss
Total Unrealized Loss		$ (1,278)
Percent to Total		10.50%
Debt securities | NAIC Rating 3 | BB | Ba
Fair value
Total Fair value		$ 4,835
Cost or amortized Cost
Total Cost or Amortized Cost		4,906
Unrealized Loss
Total Unrealized Loss		$ (71)
Percent to Total		0.60%
Debt securities | NAIC Rating 4 | B
Fair value
Total Fair value		$ 6,776
Cost or amortized Cost
Total Cost or Amortized Cost		7,299
Unrealized Loss
Total Unrealized Loss		$ (523)
Percent to Total		4.30%
Debt securities | NAIC Rating 5 | CCC or lower
Fair value
Total Fair value		$ 2,312
Cost or amortized Cost
Total Cost or Amortized Cost		2,757
Unrealized Loss
Total Unrealized Loss		$ (445)
Percent to Total		3.60%
U.S. government
Fair value
Less than 12 months		$ 46,069	48,500
Total Fair value		46,069	48,500
Cost or amortized Cost
Less than 12 months		46,545	49,079
Total Cost or Amortized Cost		46,545	49,079
Unrealized Loss
Less than 12 months		(476)	(579)
Total Unrealized Loss		(476)	(579)
Non-U.S. govt. & agency
Fair value
Less than 12 months		5,847	7,647
Total Fair value		5,847	7,647
Cost or amortized Cost
Less than 12 months		6,011	8,015
Total Cost or Amortized Cost		6,011	8,015
Unrealized Loss
Less than 12 months		(164)	(368)
Total Unrealized Loss		(164)	(368)
Mortgage-backed
Fair value
Less than 12 months		176,538	175,858
12 Months and Greater		4,201	5,737
Total Fair value		180,739	181,595
Cost or amortized Cost
Less than 12 months		179,861	179,238
12 months and greater		4,345	5,925
Total Cost or Amortized Cost		184,206	185,163
Unrealized Loss
Less than 12 months		(3,323)	(3,380)
12 Months and Greater		(144)	(188)
Total Unrealized Loss		(3,467)	(3,568)
ABS/CMBS
Fair value
Less than 12 months	[1]	37,672	48,907
12 Months and Greater	[1]	6,431	5,272
Total Fair value	[1]	44,103	54,179
Cost or amortized Cost
Less than 12 months	[1]	37,898	49,372
12 months and greater	[1]	6,581	5,364
Total Cost or Amortized Cost	[1]	44,479	54,736
Unrealized Loss
Less than 12 months	[1]	(226)	(465)
12 Months and Greater	[1]	(150)	(92)
Total Unrealized Loss	[1]	(376)	(557)
Corporate Debt
Fair value
Less than 12 months		122,221	144,353
12 Months and Greater		10,414	15,535
Total Fair value		132,635	159,888
Cost or amortized Cost
Less than 12 months		124,348	146,979
12 months and greater		11,649	18,579
Total Cost or Amortized Cost		135,997	165,558
Unrealized Loss
Less than 12 months		(2,127)	(2,626)
12 Months and Greater		(1,235)	(3,044)
Total Unrealized Loss		(3,362)	(5,670)
Municipal
Fair value
Less than 12 months		182,139	250,930
Total Fair value		182,139	250,930
Cost or amortized Cost
Less than 12 months		186,488	257,518
Total Cost or Amortized Cost		186,488	257,518
Unrealized Loss
Less than 12 months		(4,349)	(6,588)
Total Unrealized Loss		(4,349)	(6,588)
Equity securities
Fair value
Less than 12 months		9,552	7,438
12 Months and Greater		3,925	1,973
Total Fair value		13,477	9,411
Cost or amortized Cost
Less than 12 months		10,568	8,029
12 months and greater		4,644	2,648
Total Cost or Amortized Cost		15,212	10,677
Unrealized Loss
Less than 12 months		(1,016)	(591)
12 Months and Greater		(719)	(675)
Total Unrealized Loss		$ (1,735)	$ (1,266)

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