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Stock-based Compensation Expense - Summary of Restricted Stock Activity under 2016 Plan (Details) - Grants Under the 2016 Stock Incentive Plan - Restricted Stock
6 Months Ended
Jun. 30, 2019
$ / shares
shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]  
Number of Shares, Unvested, Beginning balance | shares 757,022
Number of Shares, Vested | shares (198,056)
Number of Shares, Repurchased | shares (51,373)
Number of Shares, Unvested, Ending balance | shares 507,593
Weighted Average Grant Date Fair Value, Unvested, Beginning balance | $ / shares $ 3.07
Weighted Average Grant Date Fair Value, Vested | $ / shares 3.04
Weighted Average Grant Date Fair Value, Repurchased | $ / shares 3.07
Weighted Average Grant Date Fair Value, Unvested, Ending balance | $ / shares $ 3.08