RESERVE FOR LOSSES AND LOSS EXPENSES - Prior year development (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Net favorable prior year reserve development	$ 200,054	$ 292,377	$ 243,048
Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Net favorable prior year reserve development	48,969	55,905	23,447
Reinsurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Net favorable prior year reserve development	$ 151,085	$ 236,472	$ 219,601