CASH, CASH EQUIVALENTS AND SHORT-TERM INVESTMENTS - SUMMARY (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Cash, cash equivalents, short-term and marketable securities
Cash	$ 42,106	$ 103,119
Money market funds	37	95
Commercial paper	14,241	23,670
Agency discount notes, Amortized Cost	849
Total cash and cash equivalents, Amortized Cost	57,233	126,884
Total short-term investments		1,210
Total cash, cash equivalents and short-term investments		128,094
Gross Unrealized Losses
Total short-term investments		(5)
Total cash, cash equivalents and short-term investments		(5)
Fair Value
Cash	42,106	103,119
Money market funds	37	95
Commercial paper	14,241	23,670
U.S. Agency discount notes	849
Total cash and cash equivalents	$ 57,233	126,884	$ 113,225	$ 117,709
Total short-term investments		1,205
Total cash, cash equivalents and short-term investments		128,089
Corporate debt securities and commercial paper
Cash, cash equivalents, short-term and marketable securities
Short-term investments, Maturities less than 1 year		1,210
Gross Unrealized Losses
Short-term investments, Maturities less than 1 year		(5)
Fair Value
Short-term investments, Maturities less than 1 year		$ 1,205