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Stock-Based Compensation - Additional Information (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Mar. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Stock options, grants in period, shares 1,311,750 210,808 3,087,650 1,396,989    
Stock options, grants in period, weighted average grant date fair value per share $ 6.28 $ 11.23 $ 6.63 $ 7.29    
Stock options, exercised 18,701 5,171 18,770 713,931    
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 665,301   665,301   927,123  
Cash received from early exercise of unvested options $ 2.3   $ 2.3   $ 2.8  
Award vesting period     48 months