Statements of Cash Flows - USD ($)	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Operating activities:
Net loss	$ (35,063,979)	$ (36,403,967)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	155,257	184,868
Stock-based compensation	2,348,526	1,889,932
Amortization of bond premium in marketable securities	758,337
Noncash interest expense on loans	117,188
Revaluation of warrant liability	71,102	(109,522)
Changes in operating assets and liabilities:
Prepaid expenses, offering costs and other assets	(317,720)	2,946,831
Accounts payable and accrued expenses	(1,917,189)	4,762,085
Deferred revenue	5,625,000
Net cash used in operating activities	(28,223,478)	(26,729,773)
Investing activities:
Purchase of property and equipment	(203,440)	(421,517)
Maturities of marketable securities	48,350,000
Purchases of marketable securities	(108,285,889)
Net cash provided by (used in) investing activities	(60,139,329)	(421,517)
Financing activities:
Proceeds from exercise of common stock options	683,053	101,058
Proceeds from issuance of common stock, net	90,510,810	59,534,984
Net proceeds from debt issuance		1,775,012
Capital lease payments	(1,906)	(405)
Net cash provided by financing activities	91,191,957	61,410,649
Net increase in cash and cash equivalents	2,829,150	34,259,359
Cash and cash equivalents-beginning of period	36,205,559	37,965,198
Cash and cash equivalents-end of period	$ 39,034,709	$ 72,224,557