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Description of Business - Additional Information (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 9 Months Ended 12 Months Ended
Jul. 02, 2018
Sep. 30, 2018
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Description Of Business [Line Items]          
IPO closing date Jul. 02, 2018        
Net proceeds from initial public offering     $ 99,998    
Common stock, par value per share   $ 0.01 $ 0.01   $ 0.01
Common stock, shares authorized for issuance   200,000,000 200,000,000   200,000,000
Preferred stock, shares authorized for issuance   10,000,000 10,000,000   10,000,000
Preferred stock, par value per share   $ 0.01 $ 0.01   $ 0.01
Cash and cash equivalents   $ 106,760 $ 106,760   $ 29,147
Accumulated deficit   214,900 214,900   196,946
Cash flows from operating activities     18,315 $ 11,412 $ 11,100
$35.0 Million Credit Facility | Oxford Finance LLC          
Description Of Business [Line Items]          
Borrowings outstanding under credit facility   $ 30,000 $ 30,000    
Credit facility maturity date     Mar. 01, 2022    
Common Stock          
Description Of Business [Line Items]          
Common stock shares issued and sold   6,325,000      
Convertible preferred stock converted into common stock   10,994,000      
Initial Public Offering          
Description Of Business [Line Items]          
Net proceeds from initial public offering $ 96,500        
Common stock, par value per share $ 0.01        
Common stock, shares authorized for issuance 200,000,000        
Preferred stock, shares authorized for issuance 10,000,000        
Preferred stock, par value per share $ 0.01        
Initial Public Offering | Common Stock          
Description Of Business [Line Items]          
Common stock shares issued and sold 6,325,000        
Issuance price per share $ 17.00        
Convertible preferred stock converted into common stock 11,000,000