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Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($)
$ / shares in Units, $ in Thousands
9 Months Ended
Sep. 30, 2019
Number of Shares Underlying Outstanding Options  
Outstanding at Beginning balance 4,669,000
Options granted 2,139,000
Options exercised (82,000)
Options cancelled (70,000)
Outstanding at Ending balance 6,656,000
Exercisable at End of Period 3,450,000
Weighted-Average Exercise Price  
Outstanding at Beginning balance $ 7.86
Options granted 4.74
Options exercised 2.81
Options cancelled 9.23
Outstanding at Ending balance 6.90
Exercisable at End of Period $ 7.90
Weighted Average Remaining Contractual Term (Years)  
Outstanding at Ending balance 7 years 4 months 6 days
Exercisable at End of Period 5 years 11 months 12 days
Aggregate Value Intrinsic  
Outstanding $ 177
Exercisable at End of Period $ 177