Summary of Significant Accounting Policies - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue	$ 1,255,318		$ 1,255,318
Accumulated deficit	(549,181,345)	$ 200,000	(537,375,776)
Revenue, remaining performance obligation	20,700,000
Restricted cash, current	10,850,136		10,701,305	$ 10,138,890	$ 10,138,890
Difference between Revenue Guidance in Effect before and after Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue		1,800,000
Deferred costs		2,100,000
Accounts receivable		$ 100,000
Revenue	0
Accrued Liabilities and Other Current Liabilities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue	1,300,000		$ 1,300,000
Secured Debt | Loan Agreement
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Restricted cash and cash equivalents	5,000,000
Accounting Standards Update 2016-18
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Restricted cash	$ 2,500,000