Stock-Based Compensation (Details) - USD ($)	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation			$ 8,867,810	$ 8,516,349
Total unrecognized compensation cost related to unvested stock options	$ 4,300,000		$ 4,300,000
Period over which total unrecognized compensation cost related to unvested stock options will be recognized			1 year 10 months 30 days
Weighted average grant date fair value (in dollars per share)	$ 1.65	$ 2.96	$ 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	$ 5,300,000		$ 5,300,000
Period over which total unrecognized compensation cost related to unvested stock options will be recognized			1 year 8 months
Weighted average grant date fair value, restricted stock units (in dollars per share)	$ 2.68	$ 0	$ 3.09	$ 4.31
Employees and Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	$ 1,800,000	$ 2,200,000	$ 8,100,000	$ 8,000,000
Employees and Directors \| Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.75%	2.86%	2.43%	2.73%
Expected volatility	65.00%	71.00%	70.00%	72.00%
Expected life in years	6 years 1 month	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	$ 264,000	$ 160,000	$ 744,000	$ 334,000
Research and Development Expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	922,000	1,200,000	5,100,000	4,700,000
General and Administrative Expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	$ 885,000	$ 1,000,000	$ 3,000,000	$ 3,300,000

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

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Amount of noncash expense for share-based payment arrangement.

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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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