RESERVE FOR LOSSES AND LOSS EXPENSES - Prior year development (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Net favorable prior year reserve development	$ 199,662	$ 200,054	$ 292,377
Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Net favorable prior year reserve development	92,806	60,459	48,978
Reinsurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Net favorable prior year reserve development	$ 106,856	$ 139,595	$ 243,399