RESERVE FOR LOSSES AND LOSS EXPENSES - Reserve for losses and loss expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Insurance Loss Reserves [Abstract]
Reserve for reported losses and loss expenses	$ 5,137,659	$ 3,358,514
Reserve for losses incurred but not reported	7,859,894	6,339,313
Reserve for losses and loss expenses	$ 12,997,553	$ 9,697,827	$ 9,646,285	$ 9,596,797