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Stock-Based Awards (Tables)
 12 Months Ended
Dec. 31, 2020
Stock-Based Awards [Abstract]  
Weighted-Average Assumptions for Stock Options

Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”). The table below for the years ended December 31, 2020, 2019 and 2018 is post adoption of ASU 2018-07. The weighted average assumptions that the Company used in the Black-Scholes pricing model to determine the fair value of the stock options granted to employees, non-employees and directors were as follows:

  
Years Ended December 31,
  
   
2020
    
2019
    
2018
  
             
Risk-free interest rate
    
1.00
%
    
2.26
%
    
2.64
%
Expected term (in years)
    
5.84
      
5.81
      
5.85
  
Expected volatility
    
76.98
%
    
75.71
%
    
87.70
%
Expected dividend yield
    
0.00
%
    
0.00
%
    
0.00
%
Exercise price
  
$
22.96
    
$
14.47
    
$
17.82
  
Fair value of common stock
  
$
22.96
    
$
14.47
    
$
17.98
  
Stock Option Activity

The following table summarizes stock option activity for the years ended December 31, 2020 and 2019, under the Revised 2014 Plan:

  
Number of
Shares
    
Weighted
Average
Exercise
Price
    
Weighted
Average
Contractual
Term (Years)
    
Aggregate
Intrinsic
Value
  
                         
Outstanding as of December 31, 2018
    
8,615,997
    
$
4.48
      
7.51
    
$
94,474
  
Granted
    
1,706,116
      
14.47
      
9.29
          
Exercised
    
(110,325
)
    
2.31
              
1,429
  
Cancelled
    
(448,247
)
    
10.83
                  
Outstanding as of December 31, 2019
    
9,763,541
    
$
5.95
      
6.96
    
$
164,021
  
Granted
    
2,193,546
      
22.96
      
8.97
          
Exercised
    
(586,857
)
    
4.82
              
13,494
  
Cancelled
    
(319,299
)
    
15.82
                  
Outstanding as of December 31, 2020
    
11,050,931
    
$
9.10
      
6.55
    
$
504,079
  
                                 
Options vested and exercisable as of December 31, 2020
    
8,244,686
    
$
4.98
      
5.70
    
$
409,530
  
Options unvested as of December 31, 2020
    
2,806,245
    
$
21.15
      
9.04
          
Stock-Based Compensation Expense by Award Type

Stock-based compensation expense recognized by award type is as follows:

  
Years Ended December 31,
  
   
2020
    
2019
    
2018
  
             
Stock options
  
$
18,527
    
$
13,371
    
$
13,601
  
Restricted stock units
    
40
      
-
      
-
  
Total share based compensation expense
  
$
18,567
    
$
13,371
    
$
13,601
  
Stock-Based Compensation Expense

Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive loss was as follows:

  
Years Ended December 31,
  
   
2020
    
2019
    
2018
  
             
Research and development
  
$
7,355
    
$
6,153
    
$
7,375
  
General and administrative
    
11,211
      
7,218
      
6,226
  
Total share based compensation expense
  
$
18,567
    
$
13,371
    
$
13,601
  
RSU Activity

The following table summarizes our RSU activity for the year ended December 31, 2020:

  
Number of
Shares
    
Weighted
Average
Grant Date
Fair Value
  
             
Unvested as of December 31, 2019
    
-
      
-
  
Granted
    
20,000
    
$
25.06
  
Unvested as of December 31, 2020
    
20,000