ORGANIZATION AND SIGINIFICANT ACCOUNTING POLICIES - Cash and Cash Equivalents and Restricted Cash (Details) - USD ($)	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Cash and cash equivalents	$ 135,561,000	$ 108,282,000
Restricted cash	45,532,000	17,981,000
Total cash, cash equivalents and restricted cash	$ 181,093,000	$ 126,263,000	$ 169,012,000	$ 246,712,000