INCOME TAXES - Effective tax rate (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of effective tax rate (% of income before income taxes) [Line Items]
Unrecognized tax benefits	$ 0	$ 0
Income (loss) before income taxes
Bermuda (domestic)	(188,420,000)	469,306,000	$ 652,235,000
Foreign	(188,091,000)	50,402,000	(7,576,000)
Total income before income taxes	$ (376,511,000)	$ 519,708,000	$ 644,659,000
Reconciliation of effective tax rate (% of income before income taxes)
Expected tax rate	0.00%	0.00%	0.00%
Valuation allowance	0.00%	0.20%	1.20%
Net tax exempt income	0.10%	(0.20%)	(0.10%)
Change in U.S. enacted tax rate	(11.10%)	0.00%	0.00%
Other	0.30%	0.30%	(0.90%)
Actual tax rate	2.00%	1.20%	0.50%
U.S.
Reconciliation of effective tax rate (% of income before income taxes)
Foreign taxes at local expected rates:	6.60%	(0.60%)	0.80%
Europe
Reconciliation of effective tax rate (% of income before income taxes)
Foreign taxes at local expected rates:	5.80%	1.50%	(0.20%)
Other
Reconciliation of effective tax rate (% of income before income taxes)
Foreign taxes at local expected rates:	0.30%	0.00%	(0.30%)