INVESTMENTS (Details) - USD ($) $ in Thousands		Sep. 30, 2016	Dec. 31, 2015
Amortized Cost
Due in one year or less		$ 29,101
Due after one year through five years		357,543
Due after five years through 10 years		544,117
Due after 10 years		275,190
Mtge/ABS/CMBS	[1]	418,917
Total amortized cost		1,624,868	$ 1,518,156
Fair Value
Due in one year or less		29,173
Due after one year through five years		365,930
Due after five years through 10 years		571,544
Due after 10 years		290,637
Mtge/ABS/CMBS	[1]	430,321
Total fair value		1,687,605	1,538,110
Fair value measured on recurring basis
Available-for-sale
Fair Value		2,069,887	1,913,534
Fair value measured on recurring basis | Significant Unobservable Inputs (Level 3)
INVESTMENTS
Total assets at fair value		0
Debt securities
Available-for-sale
Available-for-sale Securities, Amortized Cost Basis, Total		1,624,868	1,518,156
Gross Unrealized Gains		68,148	37,750
Gross Unrealized Losses		(5,411)	(17,796)
Fair Value		1,687,605	1,538,110
U.S. government
Available-for-sale
Available-for-sale Securities, Amortized Cost Basis, Total		82,796	43,597
Gross Unrealized Gains		1,523	58
Gross Unrealized Losses		(6)	(112)
Fair Value		84,313	43,543
U.S. Agency
Available-for-sale
Available-for-sale Securities, Amortized Cost Basis, Total		11,493	15,481
Gross Unrealized Gains		807	306
Gross Unrealized Losses			(47)
Fair Value		12,300	15,740
Non-U.S. govt. & agency
Available-for-sale
Available-for-sale Securities, Amortized Cost Basis, Total		6,037	5,035
Gross Unrealized Gains		153
Gross Unrealized Losses		(190)	(557)
Fair Value		6,000	4,478
Agency MBS
Available-for-sale
Available-for-sale Securities, Amortized Cost Basis, Total		302,761	250,060
Gross Unrealized Gains		9,603	6,451
Gross Unrealized Losses		(279)	(1,619)
Fair Value		312,085	254,892
ABS/CMBS
Available-for-sale
Available-for-sale Securities, Amortized Cost Basis, Total	[2]	116,156	91,559
Gross Unrealized Gains	[2]	2,322	995
Gross Unrealized Losses	[2]	(242)	(606)
Fair Value	[2]	118,236	91,948
Corporate
Available-for-sale
Available-for-sale Securities, Amortized Cost Basis, Total		563,045	523,351
Gross Unrealized Gains		22,093	8,565
Gross Unrealized Losses		(4,464)	(14,807)
Fair Value		580,674	517,109
Municipal
Available-for-sale
Available-for-sale Securities, Amortized Cost Basis, Total		542,580	589,073
Gross Unrealized Gains		31,647	21,375
Gross Unrealized Losses		(230)	(48)
Fair Value		573,997	610,400
Equity securities
Available-for-sale
Available-for-sale Securities, Amortized Cost Basis, Total		200,595	202,437
Gross Unrealized Gains		182,616	174,443
Gross Unrealized Losses		(929)	(1,456)
Fair Value		$ 382,282	$ 375,424

[1] Mortgage-backed, asset-backed and commercial mortgage-backed

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