v3.8.0.1
REDEEMABLE CONVERTIBLE PREFERRED STOCK - Narrative (Details)
$ / shares in Units, $ in Thousands
 12 Months Ended
Oct. 12, 2017
USD ($)
$ / shares
shares
Apr. 30, 2017
Dec. 31, 2017
USD ($)
board_member
$ / shares
shares
Dec. 31, 2016
USD ($)
shares
Dec. 31, 2015
USD ($)
shares
Dec. 31, 2014
USD ($)
shares
Temporary Equity [Line Items]            
Redeemable convertible preferred stock, authorized (in shares) | shares       7,000,000    
Redeemable convertible preferred stock, issued (in shares) | shares       5,446,978    
Conversion ratio (in shares) | shares 1   1      
Redeemable convertible preferred stock, outstanding (in shares) | shares     0      
Redeemable convertible preferred stock     $ 0 $ 71,303    
Dividend rate, percentage     8.00%      
Accretion of redeemable preferred stock to redemption value     $ 4,563 5,876 $ 4,797  
Payment of preferred stock dividends     5,965 0 0  
Issuance of common stock, net of issuance cost     54,997      
Redeemable preferred stock dividends, cash preference payment     16,000 $ 0 $ 0  
Common Stock            
Temporary Equity [Line Items]            
Issuance of common stock, net of issuance cost     $ 1      
Redeemable convertible preferred stock            
Temporary Equity [Line Items]            
Conversion ratio (in shares) | shares     1      
Terms of conversion, consent percentage     51.00%      
Terms of redemption, consent percentage   51.00%        
Series A            
Temporary Equity [Line Items]            
Redeemable convertible preferred stock, authorized (in shares) | shares     0 1,000,000    
Redeemable convertible preferred stock, issued (in shares) | shares     0 1,000,000    
Redeemable convertible preferred stock, outstanding (in shares) | shares     0 1,000,000 1,000,000 1,000,000
Redeemable convertible preferred stock     $ 0 $ 23,439 $ 21,654 $ 20,005
Dividend rate, per-dollar-amount (in dollars per share) | $ / shares     $ 16.00      
Issuance of common stock, net of issuance cost     $ 8,939      
Terms of conversion, payment     16,000      
Redeemable preferred stock dividends, cash preference payment     $ 16,000      
Preferred stock, voting rights, number of board of directors member to be elected | board_member     1      
Series B            
Temporary Equity [Line Items]            
Redeemable convertible preferred stock, authorized (in shares) | shares     0 6,000,000    
Redeemable convertible preferred stock, issued (in shares) | shares     0 4,446,978    
Redeemable convertible preferred stock, outstanding (in shares) | shares     0 4,446,978 4,446,978 4,446,978
Redeemable convertible preferred stock     $ 0 $ 47,864 $ 43,773 $ 40,625
Dividend rate, per-dollar-amount (in dollars per share) | $ / shares     $ 8.77      
Issuance of common stock, net of issuance cost     $ 5,965 $ 0 $ 0  
Terms of conversion, dividend payment percentage     50.00%      
Preferred stock, voting rights, number of board of directors member to be elected | board_member     2      
IPO            
Temporary Equity [Line Items]            
Public offering price (in dollars per share) | $ / shares $ 13.00          
IPO | Common Stock            
Temporary Equity [Line Items]            
Public offering price (in dollars per share) | $ / shares $ 13.00          
IPO | Series A | Common Stock            
Temporary Equity [Line Items]            
Issuance of common stock, net of issuance cost $ 8,900          
IPO | Series B            
Temporary Equity [Line Items]            
Payment of preferred stock dividends $ 6,000   $ 6,000