Stock-Based Compensation (Tables)
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12 Months Ended |
Dec. 31, 2018 |
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| Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07 |
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| Schedule of stock option activity |
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Weighted
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Weighted
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average
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average
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Aggregate
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Number of
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exercise
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contractual
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intrinsic
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shares
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price
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term
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value
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(in years)
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(in thousands)
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Outstanding as of December 31, 2017
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4,661,635
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$
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1.29
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8.64
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$
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23,264
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Granted
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10,405,323
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12.12
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Exercised
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(225,375)
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0.29
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Forfeited
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(56,813)
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1.44
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Outstanding as of December 31, 2018
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14,784,770
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$
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8.93
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8.89
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$
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126,367
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Vested and expected to vest as of December 31, 2018
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14,784,770
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$
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8.93
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8.89
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$
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126,367
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Options exercisable as of December 31, 2018
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2,622,981
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$
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1.74
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7.59
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$
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37,726
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| Schedule of restricted common stock award activity |
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Weighted average
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grant‑date
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Shares
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fair value
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Unvested restricted common stock as of December 31, 2017
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5,227,014
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$
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0.514
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Issued
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—
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—
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Vested
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(3,667,613)
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0.519
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Forfeited
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—
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—
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Unvested restricted common stock as of December 31, 2018
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1,559,401
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$
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0.502
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| Schedule of allocation of share based compensation |
The Company recorded stock-based compensation expense in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands):
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Year ended December 31,
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2018
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2017
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2016
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Research and development expenses
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$
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3,787
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$
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1,756
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$
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107
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General and administrative expenses
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23,741
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16,147
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40
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$
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27,528
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$
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17,903
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$
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147
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| Employees, directors and non-employees |
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| Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07 |
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| Schedule of assumptions used to determine the fair value of stock awards granted |
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Year ended December 31,
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2018
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2017
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2016
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Risk-free interest rate
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2.71
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%
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2.05
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%
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1.81
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%
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Expected volatility
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74.0
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%
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75.6
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%
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77.2
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%
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Expected dividend yield
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—
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—
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—
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Expected term (in years)
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6.21
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6.39
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6.25
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| Market-Based Stock Options |
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| Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07 |
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| Schedule of assumptions used to determine the fair value of stock awards granted |
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Year ended December 31,
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2018
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2017
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2016
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Risk-free interest rate
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3.15
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%
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—
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%
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—
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%
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Expected volatility
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69.0
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%
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—
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%
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—
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%
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Expected dividend yield
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—
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—
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—
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Derived service period (in years)
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2.30
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—
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—
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| ASU 2018-07 |
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| Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07 |
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| Schedule of assumptions used to determine the fair value of stock awards granted |
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Year ended December 31,
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2017
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2016
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Risk-free interest rate
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2.02% - 2.31%
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2.35% - 2.45%
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Expected volatility
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74% - 85%
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81% - 85%
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Expected dividend yield
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—
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—
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Expected term (in years)
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6 - 10
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8 - 10
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Fair value of common stock
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$0.19 - $6.28
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$0.18 - $0.19
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