FAIR VALUE MEASUREMENTS AND FINANCIAL INSTRUMENTS - SCHEDULE OF ASSETS AND LIABILITIES MEASURED AT FAIR VALUE ON A RECURRING BASIS (Details) - Fair Value on a Recurring Basis - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets:
Cash equivalents	$ 195,902	$ 28,064
Money market funds
Assets:
Cash equivalents	189,207	28,064
Treasury discount notes
Assets:
Cash equivalents	500
Certificates of deposit
Assets:
Cash equivalents	6,195
Level 1
Assets:
Cash equivalents	189,707	28,064
Level 1 | Money market funds
Assets:
Cash equivalents	189,207	28,064
Level 1 | Treasury discount notes
Assets:
Cash equivalents	500
Level 1 | Certificates of deposit
Assets:
Cash equivalents	0
Level 2
Assets:
Cash equivalents	6,195	0
Level 2 | Money market funds
Assets:
Cash equivalents	0	0
Level 2 | Treasury discount notes
Assets:
Cash equivalents	0
Level 2 | Certificates of deposit
Assets:
Cash equivalents	6,195
Level 3
Assets:
Cash equivalents	0	0
Level 3 | Money market funds
Assets:
Cash equivalents	0	$ 0
Level 3 | Treasury discount notes
Assets:
Cash equivalents	0
Level 3 | Certificates of deposit
Assets:
Cash equivalents	$ 0