Stock-Based Compensation (Details) - USD ($)	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation			$ 6,787,991	$ 6,164,958
Total unrecognized compensation cost related to unvested stock options	$ 6,000,000		$ 6,000,000
Period over which total unrecognized compensation cost related to unvested stock options will be recognized			2 years
Weighted average grant date fair value (dollars per share)	$ 2.44	$ 3.00	$ 2.20	$ 2.85
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation cost related to unvested stock options	$ 4,900,000		$ 4,900,000
Period over which total unrecognized compensation cost related to unvested stock options will be recognized			1 year 11 months
Weighted average grant date fair value, restricted stock units (dollars per share)	$ 3.90	$ 4.57	$ 3.42	$ 4.31
Employees and Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	$ 3,100,000	$ 2,400,000	$ 6,300,000	$ 5,800,000
Employees and Directors \| Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	2.31%	2.81%	2.43%	2.73%
Expected volatility	70.00%	71.00%	70.00%	72.00%
Expected life in years	6 years 2 months	6 years 1 month	6 years 2 months	6 years 2 months
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	$ 207,000	$ 35,000	$ 480,000	$ 174,000
Research and Development Expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	2,300,000	1,400,000	4,200,000	3,500,000
General and Administrative Expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	$ 877,000	$ 1,000,000	$ 2,100,000	$ 2,300,000

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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Amount of noncash expense for share-based payment arrangement.

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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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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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