INCOME TAXES - Effective tax rate (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of effective tax rate (% of income before income taxes) [Line Items]
Unrecognized tax benefits	$ 0	$ 0
Income (loss) before income taxes
Bermuda (domestic)	181,597,000	(188,420,000)	$ 469,306,000
Foreign	(168,062,000)	(188,091,000)	50,402,000
Total income before income taxes	$ 13,535,000	$ (376,511,000)	$ 519,708,000
Reconciliation of effective tax rate (% of income before income taxes)
Expected tax rate	0.00%	0.00%	0.00%
Valuation allowance	(13.40%)	0.00%	0.20%
Net tax exempt income	(3.30%)	0.10%	(0.20%)
Other	5.90%	0.30%	0.30%
Actual tax rate	(217.90%)	2.00%	1.20%
U.S.
Reconciliation of effective tax rate (% of income before income taxes)
Foreign taxes at local expected rates:	65.70%	6.60%	(0.60%)
Change in enacted tax rate	0.00%	(11.10%)	0.00%
Europe
Reconciliation of effective tax rate (% of income before income taxes)
Foreign taxes at local expected rates:	(289.70%)	5.80%	1.50%
Change in enacted tax rate	16.90%	0.00%	0.00%
Other
Reconciliation of effective tax rate (% of income before income taxes)
Foreign taxes at local expected rates:	0.00%	0.30%	0.00%
Foreign Tax Credit And Change In Investment Related Items
Reconciliation of effective tax rate (% of income before income taxes) [Line Items]
Change in valuation allowance	$ (1,200,000)