Stock-Based Compensation (Details) - USD ($)	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Proceeds from issuance of stock			$ 329,960,855	$ 2,296,316
Stock-based compensation			7,663,988	$ 6,787,991
Total unrecognized compensation cost related to unvested stock options	$ 9,400,000		$ 9,400,000
Period over which total unrecognized compensation cost related to unvested stock options will be recognized (in years)			2 years 3 months 18 days
Weighted average grant date fair value (in dollars per share)	$ 9.52	$ 2.44	$ 6.19	$ 2.20
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation cost related to unvested stock options	$ 15,000,000.0		$ 15,000,000.0
Period over which total unrecognized compensation cost related to unvested stock options will be recognized (in years)			2 years 3 months 18 days
Weighted average grant date fair value, restricted stock units (in dollars per share)	$ 14.50	$ 3.90	$ 9.12	$ 3.42
Research and Development Expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	$ 1,800,000	$ 2,300,000	$ 4,100,000	$ 4,200,000
General and Administrative Expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	1,600,000	877,000	3,000,000.0	2,100,000
Employees and Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	$ 3,400,000	$ 3,100,000	$ 7,100,000	$ 6,300,000
Employees and Directors \| Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	0.40%	2.31%	0.67%	2.43%
Expected volatility	80.00%	70.00%	77.00%	70.00%
Expected life in years	5 years 10 months 24 days	6 years 2 months 12 days	5 years 10 months 24 days	6 years 2 months 12 days
Dividend yield	0.00%	0.00%	0.00%	0.00%
Non Employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	$ 224,000	$ 207,000	$ 607,000	$ 480,000

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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The cash inflow from the additional capital contribution to the entity.

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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