HISTORICAL LOSS AND LAE DEVELOPMENT - Reserve Development (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	$ (65,427)	$ (64,842)	$ (72,481)
Casualty segment
(Favorable)/unfavorable reserve development
Number of years	2 years
(Favorable)/unfavorable reserve development	$ (45,654)	(52,825)	(61,805)
Casualty segment | Personal umbrella
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development		(800)	(7,500)
Casualty segment | Commercial transportation
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(5,400)		3,500
Casualty segment | Executive products
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development			(8,900)
Casualty segment | General liability
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(14,700)	(28,100)	(28,500)
Casualty segment | Runoff casualty business
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	5,400	(900)	(2,000)
Casualty segment | Commercial property
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(10,700)	(4,400)	(6,500)
Casualty segment | Miscellaneous Professional Liability Member
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development			500
Casualty segment | P&C Package
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(6,600)	(9,400)	(8,100)
Casualty segment | Professional services
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	3,200
Property segment
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(11,848)	(1,123)	(7,273)
Property segment | Recreational Vehicle
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	1,300
Property segment | Marine
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(9,200)	(5,800)	(5,900)
Property segment | Other direct property products
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development			(3,500)
Property segment | Assumed property
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(4,900)	4,000	1,400
Property segment | Crop Member
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development		1,200
Surety segment
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(7,925)	(10,894)	$ (3,403)
Surety segment | Contract surety product
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(2,200)	(4,600)
Surety segment | Energy surety product
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(2,000)	(1,200)
Surety segment | Commercial surety product
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(4,000)	(4,300)
Surety segment | Miscellaneous Surety Product
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	$ 300	$ (900)