SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Insurance Risk (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Unpaid losses and settlement expenses	$ 1,271,503	$ 1,139,337	$ 1,103,785	$ 1,121,040