Liquidity and Capital Resources (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Estimated Projected Expenditure [Line Items]
Cash and cash equivalents	$ 35,097,042	$ 37,643,404	$ 48,003,315	$ 56,959,485
Projected expenditures for clinical programs	25,700,000
Contract Liabilities [Member]
Estimated Projected Expenditure [Line Items]
Projected expenditures for clinical programs	1,200,000
Freiburg Operation [Member]
Estimated Projected Expenditure [Line Items]
Projected expenditures for clinical programs	3,600,000
General Operation [Member]
Estimated Projected Expenditure [Line Items]
Projected expenditures for clinical programs	300,000
Other General and Administrative Expenses [Member]
Estimated Projected Expenditure [Line Items]
Projected expenditures for clinical programs	9,500,000
Interest and Principal Payments [Member]
Estimated Projected Expenditure [Line Items]
Projected expenditures for clinical programs	$ 11,100,000