CASH, CASH EQUIVALENTS AND SHORT-TERM INVESTMENTS - Summary of Securities Classifies as Cash and Cash Equivalents and Short-term Investments (Details) - Cash and cash equivalents - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 109,694	$ 110,949
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(3)	0
Fair Value	109,691	110,949
Cash
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	94,311	95,660
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	94,311	95,660
Money market funds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	41	11
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	41	11
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	14,594	1,250
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(3)	0
Fair Value	14,591	1,250
U.S. Treasury Securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	748	14,028
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 748	$ 14,028