SIGNIFICANT ACCOUNTING POLICIES AND RECENT ACCOUNTING STANDARDS - Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Interest-bearing deposits, interest rate	1.00%
Restricted cash deposits, interest rate	0.90%
Cash and cash equivalents	$ 50,468	$ 21,262	$ 40,335	$ 22,838
Restricted cash	20,000	0	0	130
Cash, cash equivalents and restricted cash	$ 70,468	$ 21,262	$ 40,335	$ 22,968