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Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($)
 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Cash flows from operating activities:    
Net loss $ (31,685,295) $ (21,641,308)
Adjustments to reconcile net loss to net cash used in operating activities:    
Depreciation and amortization 9,310 12,959
Amortization of debt discount recorded as interest expense 170,822 270,944
(Accretion) amortization of marketable securities (83,268) 102,632
Stock-based compensation expense 3,775,469 2,558,456
Changes in operating assets and liabilities    
Prepaid expenses and other current assets (51,840) 473,537
Accounts payable 35,071 (527,340)
Accrued expenses and other current liabilities 1,206,592 (1,543,783)
Accrued collaborative expenses (2,547,952) 3,533,788
Deferred revenue 41,175,600  
Net cash provided by (used in) operating activities 12,004,509 (16,760,115)
Cash flows from investing activities:    
Proceeds from the maturity and redemption of marketable securities 27,390,000 17,818,000
Purchase of marketable securities (134,554,390)  
Net cash (used in) provided by investing activities (107,164,390) 17,818,000
Cash flows from financing activities:    
Proceeds from sale of common stock in public offering 44,562,500 57,499,995
Costs paid in connection with public offering (2,944,168) (3,832,004)
Repurchase of common stock (389)  
Proceeds from exercise of common stock warrants 9,356,833 22,222,494
Proceeds from exercise of stock options 1,130,544 9,861
Repayments of notes payable (3,671,300) (389,203)
Proceeds from sale of common stock in private placement   999,999
Net cash provided by financing activities 48,434,020 76,511,142
Net (decrease) increase in cash and cash equivalents (46,725,861) 77,569,027
Cash and cash equivalents    
Beginning of period 82,980,609 14,284,054
End of period 36,254,748 91,853,081
Supplemental disclosure of cash flow information    
Cash paid for interest $ 360,272 $ 528,750