v3.8.0.1
SIGNIFICANT ACCOUNTING POLICIES - Basis of Presentation and Costs Related to the Initial Public Offering (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Oct. 12, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accounting Policies [Abstract]        
Accumulated deficit   $ 103,066 $ 78,134  
Class of Stock [Line Items]        
Payment of preferred stock dividends   5,965 0 $ 0
Redeemable preferred stock dividends, cash preference payment   $ 16,000 0 0
Conversion ratio (in shares) 1 1    
Initial public offering costs   $ 0 $ 1,979 $ 0
IPO        
Class of Stock [Line Items]        
Public offering price (in dollars per share) $ 13.00      
Initial public offering costs, capitalized $ 1,840      
Common Stock        
Class of Stock [Line Items]        
Issuance of common stock, net of issuance cost (in shares)   4,600,000    
Common Stock | IPO        
Class of Stock [Line Items]        
Issuance of common stock, net of issuance cost (in shares) 4,600,000      
Public offering price (in dollars per share) $ 13.00      
Gross proceeds $ 59,800      
Net proceeds on sale of stock 46,900      
Underwriting discounts and commissions 4,200      
Offering costs 2,700      
Series A        
Class of Stock [Line Items]        
Redeemable preferred stock dividends, cash preference payment   $ 16,000    
Series B | IPO        
Class of Stock [Line Items]        
Payment of preferred stock dividends $ 6,000 6,000    
General and Administrative Expense | Restricted stock        
Class of Stock [Line Items]        
Non-cash expense related to accelerated vesting of restricted stock   $ 2,000