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Summary of Significant Accounting Policies (Tables)
 12 Months Ended
Dec. 31, 2018
Accounting Policies [Abstract]  
Schedule of Earnings Per Share, Basic and Diluted
The following tables reconcile the components of the numerator and denominator included in the calculations of diluted net loss per share:
 
Year Ended December 31,
 
2018
 
2017
 
2016
Numerator
 
 
 
 
 
Net loss (numerator for use in basic net loss per share)
$
(96,967,830
)
 
$
(88,205,772
)
 
$
(73,740,339
)
Adjustment for decrease in fair value of warrant liability
—

 
(806,819
)
 
—

Numerator for use in diluted net loss per share
$
(96,967,830
)
 
$
(89,012,591
)
 
$
(73,740,339
)
 
 
 
 
 
 
Denominator
 
 
 
 
 
Weighted average number of common shares outstanding (denominator for use in basic net loss per share)
92,539,997

 
81,777,493

 
73,214,766

Effect of dilutive potential common shares
—

 
140,529

 
—

Denominator for use in diluted net loss per share
92,539,997

 
81,918,022

 
73,214,766

 
 
 
 
 
 
Net loss per share, diluted
$
(1.05
)
 
$
(1.09
)
 
$
(1.01
)
 
 
 
 
 
 
Net loss per share, basic
$
(1.05
)
 
$
(1.08
)
 
$
(1.01
)
Summary of common shares that were excluded from the diluted net loss per share calculation because of their anti-dilutive effect
The following table summarizes potential shares of common stock that were excluded from diluted net loss per share calculation because of their anti-dilutive effect:
 
 
Year Ended December 31,
 
2018
 
2017
 
2016
Options to purchase common stock
8,752,677

 
7,694,870

 
6,806,183

Warrants to purchase common stock
—

 
—

 
284,091

Restricted stock units
1,688,017

 
1,234,168

 
798,834

Convertible preferred stock
8,456

 
8,456

 
8,456

Total
10,449,150

 
8,937,494

 
7,897,564
Summary of assumptions used to estimate the fair value of stock options
The weighted average assumptions used in the Black-Scholes model for option grants to employees and directors are presented below:
 
 
Year Ended December 31,
 
2018
 
2017
 
2016
Risk-free interest rate
2.73%
 
2.20%
 
0.91%
Expected volatility
72%
 
73%
 
76%
Expected life in years
6
 
6
 
5
Dividend yield
—
 
—
 
—
Forfeiture rate
N/A
 
N/A
 
7%
Assumptions used in the Black-Scholes model for non-employees are presented below:
 
 
Year Ended December 31,
 
2018
 
2017
 
2016
Risk-free interest rate
2.6%-3.0%
 
2.4%-2.6%
 
2.3%-2.5%
Expected volatility
68%-101%
 
97%-102%
 
71%-104%
Expected life in years
9-10
 
9-10
 
7-10
Dividend yield
—
 
—
 
—
Schedule of New Accounting Pronouncements and Changes in Accounting Principles
The following table illustrates the impact that adopting Topic 606 has had on the Company's reported results in the consolidated statements of operations for the year ended December 31, 2018.
 
Balances Without Adoption of Topic 606 for the Year Ended December 31, 2018
 
Impact of Adopting Topic 606
 
As Reported for the Year Ended December 31, 2018
Revenues:
 
 
 
 
 
Revenue under collaborative research and development arrangements
$
29,860,785

 
$
—

 
$
29,860,785

Revenue under collaborative research and development arrangements with affiliated entities
449,524

 
—

 
449,524

Grants and miscellaneous revenue
6,315,116

 
(6,143,528
)
 
171,588

Grants and miscellaneous revenue from affiliated entity
3,348,177

 
(3,348,177
)
 
—

Total revenues
39,973,602

 
(9,491,705
)
 
30,481,897

Operating expenses:
 
 
 
 
 
Research and development
104,749,581

 
(9,491,705
)
 
95,257,876

General and administrative
29,315,159

 
—

 
29,315,159

Total operating expenses
$
134,064,740

 
$
(9,491,705
)
 
$
124,573,035

The cumulative effect of the changes made to the Company's consolidated balance sheet as of January 1, 2018 for the adoption of ASU No. 2016-01 are included in the table below:
Equity:
Balance at December 31, 2017
 
Adjustments due to ASU No. 2016-01
 
Balance at January 1, 2018
Accumulated deficit
$
(523,356,317
)
 
$
231,366

 
$
(523,124,951
)
Accumulated other comprehensive loss
$
(117,005
)
 
$
(231,366
)
 
$
(348,371
)