SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Cash and cash equivalents	$ 907,530	$ 85,688
Restricted cash and cash equivalents – current	10,952	0
Restricted cash and cash equivalents – non-current	4,000	4,144
Cash, cash equivalents and restricted cash and cash equivalents	$ 922,482	$ 89,832	$ 141,359	$ 173,956