Quarterly report pursuant to Section 13 or 15(d)

Nature of Business (Details)

v3.8.0.1
Nature of Business (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 9 Months Ended 12 Months Ended
Apr. 26, 2017
Jul. 06, 2016
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Jul. 07, 2016
Initial Public Offering                    
Common stock issued upon conversion of convertible preferred stock (in shares)   15,988,800                
Common stock, shares authorized (in shares)   200,000,000 200,000,000   200,000,000   200,000,000      
Preferred stock, shares authorized (in shares)   10,000,000 10,000,000   10,000,000   10,000,000      
Par value, common stock (in dollars per share)   $ 0.001 $ 0.001   $ 0.001   $ 0.001      
Par value, preferred stock (in dollars per share)     $ 0.001   $ 0.001   $ 0.001      
Net loss     $ 13,825 $ 14,169 $ 38,708 $ 36,778 $ 47,700 $ 29,800 $ 13,400  
Accumulated deficit     139,964   139,964   $ 101,256      
Cash equivalents and marketable securities     $ 81,900   $ 81,900          
Convertible Preferred Stock                    
Initial Public Offering                    
Convertible preferred stock, shares outstanding (in shares)                   0
Common Stock | IPO                    
Initial Public Offering                    
Shares issued   4,600,000                
Share price (in dollars per share)   $ 12.50                
Aggregate gross proceeds   $ 57,500                
Net proceeds   49,900                
Underwriting discounts and commissions and offering costs   $ 7,600                
Common Stock | Over-allotment                    
Initial Public Offering                    
Shares issued   600,000                
Common Stock | Private Placement                    
Initial Public Offering                    
Shares issued 2,592,591                  
Share price (in dollars per share) $ 13.50                  
Aggregate gross proceeds $ 35,000                  
Placement agent fees 2,100                  
Other offering expenses $ 300