CASH EQUIVALENTS AND SHORT-TERM INVESTMENTS - Cash Equivalents and Short-term Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Cash equivalents:
Cash equivalents, Amortized Cost	$ 33,436
Cash equivalents	33,436
Short-term investments:
Total, Amortized cost	46,654
Gross Unrealized Gain (Loss)	(61)
Total, Fair Value	46,593
Total cash equivalents and short-term investments, Amortized Cost	80,090
Total cash equivalents and short-term investments, Gross Unrealized Gain (Loss)	(61)
Total cash equivalents and short-term investments, Fair Value	80,029
U.S. treasuries
Short-term investments:
Total, Amortized cost	15,988
Gross Unrealized Gain (Loss)	(21)
Total, Fair Value	15,967
Corporate securities
Short-term investments:
Total, Amortized cost	23,679
Gross Unrealized Gain (Loss)	(40)
Total, Fair Value	23,639
Commercial paper
Short-term investments:
Total, Amortized cost	6,987
Gross Unrealized Gain (Loss)	0
Total, Fair Value	6,987
Money market funds
Cash equivalents:
Cash equivalents, Amortized Cost	33,436
Cash equivalents	$ 33,436	$ 80,240