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Stock-Based Compensation - Additional Information (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Stock options, grants in period, shares 1,775,900 1,186,181  
Stock options, grants in period, weighted average grant date fair value per share $ 10.36 $ 6.66  
Stock options, exercised 69 708,760  
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 811,300   927,123
Cash received from early exercise of unvested options $ 2.6   $ 2.8
Award vesting period 48 months