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Statements of Cash Flows - USD ($)
 3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Operating activities    
Net loss $ (7,554,978) $ (7,109,129)
Adjustments to reconcile net loss to net cash used in operating activities:    
Depreciation 47,234 44,245
Stock-based compensation expense 727,547 587,514
Changes in assets and liabilities    
Prepaid expenses and other current assets (241,555) 139,169
Accounts payable 1,007,159 299,464
Accrued expenses (3,221,593) (2,637,283)
Deferred rent 6,496 (29,047)
Net cash used in operating activities (9,229,690) (8,705,067)
Investing activities    
Purchases of property and equipment (17,270) (2,339)
Net cash used in investing activities (17,270) (2,339)
Financing activities    
Proceeds from issuance of common stock, net of issuance costs 1,238,619  
Proceeds from exercise of stock options 35,891 75,377
Net cash provided by financing activities 1,274,510 75,377
Net change in cash and cash equivalents (7,972,450) (8,632,029)
Cash and cash equivalents, beginning of period 46,802,560 55,198,923
Cash and cash equivalents, end of period 38,830,110 46,566,894
Supplemental disclosure of non-cash investing activities:    
Property asset acquisition included in accounts payable   $ 10,000
Supplemental disclosure of non-cash financing activities:    
Issuance costs associated with the at-the-market financing included in accounts payable and accrued expenses $ 150,767