CASH, CASH EQUIVALENTS AND SHORT-TERM INVESTMENTS - Summary of Securities Classifies as Cash and Cash Equivalents and Short-term Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Cash and cash equivalents
Cash and cash equivalents	$ 121,904	$ 126,884	$ 107,585	$ 117,709
Short-term investments
Corporate debt securities and commercial paper with maturities less than 1 year, Amortized Cost		1,210
Gross Unrealized Gains		0
Gross Unrealized Losses		(5)
Total short-term investments, Fair Value		1,205
Cash and cash equivalents, and short-term investments, Amortized Cost		128,094
Cash and cash equivalents, and short-term investments, fair value		128,089
Cash
Cash and cash equivalents
Cash and cash equivalents	106,702	103,119
Money market funds
Cash and cash equivalents
Cash and cash equivalents	13	95
Commercial paper
Cash and cash equivalents
Cash and cash equivalents	$ 15,189	$ 23,670