FAIR VALUE MEASUREMENTS AND FINANCIAL INSTRUMENTS - Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets:
Cash equivalents	$ 244,317	$ 195,902
Quoted market prices in active markets for identical assets (level 1)
Assets:
Cash equivalents	243,357	189,707
Significant other observable inputs (level 2)
Assets:
Cash equivalents	960	6,195
Significant unobservable inputs (level 3)
Assets:
Cash equivalents	0	0
Money market funds
Assets:
Cash equivalents	243,357	189,207
Money market funds | Quoted market prices in active markets for identical assets (level 1)
Assets:
Cash equivalents	243,357	189,207
Money market funds | Significant other observable inputs (level 2)
Assets:
Cash equivalents	0	0
Money market funds | Significant unobservable inputs (level 3)
Assets:
Cash equivalents	0	0
Treasury discount notes
Assets:
Cash equivalents		500
Treasury discount notes | Quoted market prices in active markets for identical assets (level 1)
Assets:
Cash equivalents		500
Treasury discount notes | Significant other observable inputs (level 2)
Assets:
Cash equivalents		0
Treasury discount notes | Significant unobservable inputs (level 3)
Assets:
Cash equivalents		0
Certificates of deposit
Assets:
Cash equivalents	960	6,195
Certificates of deposit | Quoted market prices in active markets for identical assets (level 1)
Assets:
Cash equivalents	0	0
Certificates of deposit | Significant other observable inputs (level 2)
Assets:
Cash equivalents	960	6,195
Certificates of deposit | Significant unobservable inputs (level 3)
Assets:
Cash equivalents	$ 0	$ 0