HISTORICAL LOSS AND LAE DEVELOPMENT - Reserve Development (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	$ (38,868)	$ (41,994)	$ (65,427)
Casualty segment
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(17,462)	(32,401)	(45,654)
Property segment
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(12,134)	(4,793)	(11,848)
Property segment | Marine
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(6,800)	(2,100)	(9,200)
Property segment | Assumed property
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development		(2,500)	(4,900)
Property segment | Recreational Vehicle
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development			1,300
Property segment | Commercial property
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(3,200)
Property segment | Other direct property products
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development		200
Surety segment
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(9,272)	(4,800)	(7,925)
Surety segment | Commercial surety product
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(3,500)	(1,700)	(4,000)
Surety segment | Contract surety product
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(4,400)	(100)	(2,200)
Surety segment | Energy surety product
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(1,500)	(1,900)	(2,000)
Surety segment | Miscellaneous Surety Product
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	100	(1,100)	300
Primary Occurrence | Casualty segment | General liability
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(4,600)	(17,600)	(15,400)
Primary Occurrence | Casualty segment | P&C Package
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(3,200)	(6,200)	(6,600)
Primary Occurrence | Casualty segment | Professional services
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development			3,200
Excess Occurrence | Casualty segment | Commercial products
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(9,900)	(13,800)	(10,700)
Excess Occurrence | Casualty segment | Personal umbrella
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(1,100)	4,900
Excess Occurrence | Casualty segment | Runoff casualty business
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development			5,400
Claims Made | Casualty segment
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development			(3,900)
Claims Made | Casualty segment | Executive products
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(4,400)	(14,700)
Claims Made | Casualty segment | Medical professional liability
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	3,700
Claims Made | Casualty segment | Miscellaneous Professional Liability Member
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development		(800)
Transportation | Casualty segment | Transportation
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	$ 7,400	$ 15,400	$ (5,400)