Quarterly report pursuant to Section 13 or 15(d)

Equity Incentive Plans, Employee Stock Purchase Plan and Stock-Based Compensation - Summary of Option Activity (Details)

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Equity Incentive Plans, Employee Stock Purchase Plan and Stock-Based Compensation - Summary of Option Activity (Details) - 2004 and 2018 Equity Incentive Plan - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2020
Dec. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Stock options outstanding, Beginning Balance, Shares 3,872,664  
Granted, Shares 1,304,000  
Exercised, Shares (12,937)  
Canceled and forfeited, Shares (18,092)  
Stock options outstanding, Ending Balance, Shares 5,145,635 3,872,664
Stock options exercisable, Shares 1,940,215  
Stock options outstanding, Weighted - Average Exercise Price, Beginning Balance $ 12.89  
Weighted - Average Exercise Price, Granted 8.78  
Weighted - Average Exercise Price, Exercised 4.35  
Weighted - Average Exercise Price, Canceled and forfeited 12.36  
Stock options outstanding, Weighted - Average Exercise Price, Ending Balance 11.87 $ 12.89
Stock options exercisable, Weighted - Average Exercise Price, Exercisable $ 12.60  
Weighted - Average Remaining Contract Term 8 years 2 months 19 days 7 years 10 months 17 days
Weighted - Average Remaining Contract Term, Exercisable 6 years 8 months 12 days  
Aggregate Intrinsic Value, Balance $ 3,549 $ 2,119
Aggregate Intrinsic Value, Exercisable $ 1,172