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	$ 2,131,632	$ 1,630,809	$ 1,329,187	$ 1,481,447
Restricted cash included in other current assets	1,633	2,873	20,464	126
Restricted cash included in other assets	420	0	10,548	20,100
Total cash, cash equivalents and restricted cash as shown on the consolidated statement of cash flows	$ 2,133,685	$ 1,633,682	$ 1,360,199	$ 1,501,673