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	$ 336,984	$ 221,521	$ 36,377	$ 2,462
Restricted cash included in other current assets	1,417	0	0	0
Restricted cash included in other assets	420	0	10,548	0
Total cash, cash equivalents, and restricted cash as shown on the consolidated and combined statement of cash flows	$ 338,821	$ 221,521	$ 46,925	$ 2,462