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Subsequent Events - Narrative (Details) - $ / shares
1 Months Ended 3 Months Ended 12 Months Ended
Feb. 28, 2019
Mar. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Jun. 30, 2019
May 01, 2019
Apr. 30, 2019
Apr. 03, 2019
Sep. 30, 2018
Subsequent Event [Line Items]                    
Shares of common stock reserved for issuance (in shares)   32,950   629,716 1,375,329          
Exercise price (in USD per share)   $ 11.29   $ 6.51 $ 6.54          
Shares authorized (in shares)   29,879,220 29,879,220 29,879,220            
Common stock, par value (in USD per share)   $ 0.001 $ 0.001 $ 0.001            
IPO                    
Subsequent Event [Line Items]                    
Stock issued, price per share (in USD per share)   $ 20.00 $ 20.00              
Term Loan                    
Subsequent Event [Line Items]                    
Stated interest rate                   10.75%
Interest rate paid in cash                   8.00%
Interest rate paid-in-kind                   2.75%
Employee Stock Option                    
Subsequent Event [Line Items]                    
Shares of common stock reserved for issuance (in shares)     575,314 1,371,626            
2007 Stock Option Plan | Employee Stock Option                    
Subsequent Event [Line Items]                    
Shares of common stock reserved for issuance (in shares) 32,950                  
Exercise price (in USD per share) $ 11.29                  
Grant date fair value (in USD per share) $ 160,000                  
Requisite service period 4 years                  
2019 Stock Option Plan                    
Subsequent Event [Line Items]                    
Shares of common stock reserved for issuance (in shares)   2,317,000                
Subsequent Event                    
Subsequent Event [Line Items]                    
Shares authorized (in shares)               100,000,000    
Common stock, par value (in USD per share)               $ 0.001    
Preferred shares authorized (in shares)               5,000,000    
Preferred shares (in USD per share)               $ 0.001    
Subsequent Event | IPO                    
Subsequent Event [Line Items]                    
Stock issued, price per share (in USD per share)               $ 20.00    
Subsequent Event | Term Loan                    
Subsequent Event [Line Items]                    
Stated interest rate             10.00%      
Interest rate paid in cash             8.00%      
Interest rate paid-in-kind             2.00%      
Subsequent Event | 2019 Stock Option Plan                    
Subsequent Event [Line Items]                    
Shares of common stock reserved for issuance (in shares)           36,982     687,176  
Weighted average exercise price (in dollars per share)           $ 47.54