SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Cash and cash equivalents	$ 406,493	$ 267,109
Restricted cash included in:
Restricted cash included in other current assets	7,649	26,096
Restricted cash included in other assets	16,192	18,095
Total cash, cash equivalents and restricted cash in the Statement of Cash Flows	$ 430,334	$ 311,300	$ 855,726	$ 761,865