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Stock-Based Compensation - Additional Information (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Mar. 31, 2018
Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Stock options, grants in period, shares 130,000 147,955 1,526,989 1,014,537    
Stock options, grants in period, weighted average grant date fair value per share $ 9.10 $ 1.64 $ 7.45 $ 1.74    
Common stock issued under equity compensation plans 0 113,867 713,931 632,926    
2018 Equity Incentive Plan            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Number of shares issued or transferred         3,570,000  
2015 Stock Plan            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Shares not vested and subject to repurchase 1,050,103   1,050,103     812,769
Cash received from early exercise of unvested options $ 3.1   $ 3.1     $ 0.9
Award vesting period     48 months