Quarterly report pursuant to Section 13 or 15(d)

Equity Incentive Plans - Additional Information (Details)

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Equity Incentive Plans - Additional Information (Details)
1 Months Ended 9 Months Ended 12 Months Ended
Jan. 01, 2018
shares
May 31, 2017
shares
Sep. 30, 2018
USD ($)
Participant
$ / shares
shares
Dec. 31, 2017
Restricted Stock Unit        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Number of Units, Granted     92,883  
Weighted average Grant Date Fair Value, Granted | $ / shares     $ 17.87  
Restricted Stock Unit | Director        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Number of Units, Granted     17,881  
Weighted average Grant Date Fair Value, Granted | $ / shares     $ 28.45  
Restricted Stock Unit | Vesting Rate, Year One        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Granted restricted stock units of common stock, vesting percentage     35.00%  
Restricted Stock Unit | Vesting Rate, Year Two        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Granted restricted stock units of common stock, vesting percentage     21.667%  
Restricted Stock Unit | Vesting Rate, Year Three        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Granted restricted stock units of common stock, vesting percentage     21.667%  
Restricted Stock Unit | Vesting Rate, Year Four        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Granted restricted stock units of common stock, vesting percentage     21.667%  
2017 Equity Incentive Plan        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Common stock reserved for issuance   2,413,659    
Number of shares available for future grant   900,000 1,263,076  
Shares of common stock reserved for issuance, automatic annual increase initiation date   Jan. 01, 2018    
Shares of common stock reserved for issuance, automatic annual increase end date   Jan. 01, 2027    
Percentage of number of shares of common stock outstanding       5.00%
Increase in common stock reserved for issuance 491,356      
2017 Equity Incentive Plan | Shares under 2005 Plan that would have otherwise been returned to 2005 Plan | Maximum        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Common stock reserved for issuance   1,314,752    
2017 Employee Stock Purchase Plan        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Common stock reserved for issuance     200,000  
Number of shares available for future grant     150,000  
Percentage of number of shares of common stock outstanding       2.00%
Increase in common stock reserved for issuance 150,000      
Percentage of fair market value of common stock on purchase date   85.00%    
Percentage of fair market value of common stock on purchase date of the first day of IPO   85.00%    
Purchase of common stock under ESPP, Description     The 2017 ESPP allows eligible employees to purchase shares of the Company’s common stock in an offering at a discount of the then-current trading price, up to the lesser of (1) 85% of the fair market value of the common stock on the first day of the IPO or (2) 85% of the fair market value of the common stock on the purchase date. The 2017 ESPP permits the maximum discounted purchase price permitted under U.S. tax rules, including a “lookback.”  
Initial offering period in length     24 months  
Number of participant have the right to purchase shares of common stock | Participant     0  
Fair Market Value Of Common Stock | $     $ 25,000  
Shares issued under ESPP     43,624