FAIR VALUE MEASUREMENTS AND FINANCIAL INSTRUMENTS - Assets Measured at Fair Value on a Recurring Basis (Details) - Fair Value on a Recurring Basis - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets:
Cash equivalents	$ 213,092	$ 195,902
Money market funds
Assets:
Cash equivalents	208,295	189,207
Treasury discount notes
Assets:
Cash equivalents		500
Certificates of deposit
Assets:
Cash equivalents	4,797	6,195
Quoted market prices in active markets for identical assets (level 1)
Assets:
Cash equivalents	208,295	189,707
Quoted market prices in active markets for identical assets (level 1) | Money market funds
Assets:
Cash equivalents	208,295	189,207
Quoted market prices in active markets for identical assets (level 1) | Treasury discount notes
Assets:
Cash equivalents		500
Quoted market prices in active markets for identical assets (level 1) | Certificates of deposit
Assets:
Cash equivalents	0	0
Significant other observable inputs (level 2)
Assets:
Cash equivalents	4,797	6,195
Significant other observable inputs (level 2) | Money market funds
Assets:
Cash equivalents	0	0
Significant other observable inputs (level 2) | Treasury discount notes
Assets:
Cash equivalents		0
Significant other observable inputs (level 2) | Certificates of deposit
Assets:
Cash equivalents	4,797	6,195
Significant unobservable inputs (level 3)
Assets:
Cash equivalents	0	0
Significant unobservable inputs (level 3) | Money market funds
Assets:
Cash equivalents	0	0
Significant unobservable inputs (level 3) | Treasury discount notes
Assets:
Cash equivalents		0
Significant unobservable inputs (level 3) | Certificates of deposit
Assets:
Cash equivalents	$ 0	$ 0