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STOCK INCENTIVE PLANS - Narrative (Details) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 3 Months Ended
Mar. 31, 2015
Mar. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted-average grant date fair value of employee stock options granted (in dollars per share)   $ 3.02  
Grant date fair value of employee stock options vested   $ 1,200  
Total unrecognized share-based compensation expense related to unvested employee stock options   $ 8,400  
Period to recognize unrecognized compensation cost   2 years 3 months 15 days  
ESPP      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Automatic increase in shares of stock available (as a percent) 1.00%    
Price of stock option as percentage of estimated fair value of shares on date of grant 85.00%    
Limit on employee's payroll deductions as a percentage of eligible compensation 15.00%    
Issued pursuant to the ESPP (in shares)   0  
Period to recognize unrecognized compensation cost   10 months 21 days  
Estimated unrecognized share-based compensation expense   $ 400  
Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period   4 years  
Cash received from early exercise of stock options   $ 12 $ 21
Subject to repurchase | Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unvested shares subject to repurchase (in shares)   5,341  
Maximum | ESPP      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Automatic increase in number of shares available for issuance (in shares) 490,336    
2015 EIP      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Automatic increase in shares of stock available (as a percent) 4.00%    
Term for stock options to be granted 10 years    
Price of stock option as percentage of estimated fair value of shares on date of grant 100.00%    
Voting power threshold (as a percent) 10.00%    
2015 EIP | More than 10% of voting power      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Price of stock option as percentage of estimated fair value of shares on date of grant 110.00%    
2015 EIP | Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period 3 years    
2015 EIP | Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period 4 years