COMMITMENTS AND CONTINGENCIES Letters of Credit (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Aggregate amount of letters of credit issued	$ 188,500	$ 255,400
Total equity securities, restricted cash, and cash and cash equivalents fair value pledged as security against the letters of credit	200,400	310,900
Collateral Held in Trust	377,900	$ 86,400
Financial Standby Letter of Credit
Line of Credit Facility [Line Items]
Facility	600,000
Financial Standby Letter of Credit | Butterfield Bank (Cayman) Limited
Line of Credit Facility [Line Items]
Facility	$ 100,000
Notice period required for termination	90 days
Financial Standby Letter of Credit | Citibank Europe plc
Line of Credit Facility [Line Items]
Facility	$ 400,000
Notice period required for termination	120 days
Financial Standby Letter of Credit | JP Morgan Chase Bank N.A.
Line of Credit Facility [Line Items]
Facility	$ 100,000
Line of Credit Facility, Increase (Decrease), Other, Net	$ 100,000
Notice period required for termination	120 days