CONDENSED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($)	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Cash flows from operating activities:
Net loss	$ (29,211,643)	$ (11,054,955)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	165,830	64,856
Stock-based compensation expense	3,662,708	2,627,010
Fair value adjustment on warrant liability	(564,570)	(12,234,301)
Net foreign exchange loss	0	(64)
Changes in assets and liabilities:
Receivables	(3,155,714)	(1,480,067)
Interest receivable	90,431	(48,929)
Prepaid expenses and other current assets	2,027,993	1,410,893
Accounts payable	(1,332,633)	(160,032)
Accrued expenses and other current liabilities	4,382,343	3,832,235
Net cash used in operating activities	(23,935,255)	(17,043,354)
Cash flows from investing activities:
Purchase of short-term investments	(17,960,256)	(45,601,506)
Proceeds from the sale of short-term investments	48,579,636	22,084,980
Purchases of equipment and furnishings	(69,294)	(20,733)
Net cash provided by (used in) investing activities	30,550,086	(23,537,259)
Cash flows from financing activities:
Net proceeds from public offering	0	80,535,401
Proceeds from issuance of restricted stock to employee	0	100
Net proceeds from exercise of warrants and stock options	0	428,891
Net cash provided by financing activities	0	80,964,392
Net increase in cash and cash equivalents	6,614,831	40,383,779
Cash and cash equivalents at beginning of period	32,218,905	11,483,112
Cash and cash equivalents at end of period	38,833,736	51,866,891
Supplemental disclosure of cash flow information:
Equipment and furnishings purchased on credit	5,022	107,506
Cashless warrant exercises	3	133
Cash paid for income taxes	$ 800	$ 59,431