HISTORICAL LOSS AND LAE DEVELOPMENT - Reserve Development (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	$ (49,950)	$ (38,868)	$ (41,994)
Casualty segment
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(33,252)	(17,462)	(32,401)
Property segment
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(10,813)	(12,134)	(4,793)
Property segment | Marine
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(5,000)	(6,800)	(2,100)
Property segment | Assumed property
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(2,800)		(2,500)
Property segment | Commercial property
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(2,000)	(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	(5,885)	(9,272)	(4,800)
Surety segment | Commercial surety product
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(4,600)	(3,500)	(1,700)
Surety segment | Contract surety product
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development		(4,400)	(100)
Surety segment | Energy surety product
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(1,700)	(1,500)	(1,900)
Surety segment | Miscellaneous Surety Product
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	800	100	(1,100)
Primary Occurrence | Casualty segment
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(15,600)
Primary Occurrence | Casualty segment | General liability
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(6,700)	(4,600)	(17,600)
Primary Occurrence | Casualty segment | P&C Package
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development		(3,200)	(6,200)
Excess Occurrence | Casualty segment
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(21,400)
Excess Occurrence | Casualty segment | Commercial products
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	(10,800)	(9,900)	(13,800)
Excess Occurrence | Casualty segment | Personal umbrella
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development		(1,100)	4,900
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
(Favorable)/unfavorable reserve development
(Favorable)/unfavorable reserve development	$ (500)	$ 7,400	$ 15,400