CONDENSED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($)	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Cash flows from operating activities:
Net loss	$ (36,284,995)	$ (16,680,203)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	240,391	95,834
Loss on retirement of fixed assets	0	975
Stock-based compensation expense	4,520,307	3,402,706
Fair value adjustment on warrant liability	(4,079,748)	(19,560,350)
Net foreign exchange loss	0	(29,131)
Changes in assets and liabilities:
Receivables	351,388	(3,377,135)
Interest receivable	97,873	(67,079)
Prepaid expenses and other current assets	1,545,955	251,075
Accounts payable	(447,430)	1,409,926
Accrued expenses and other current liabilities	(26,763)	5,788,188
Net cash used in operating activities	(34,083,022)	(28,765,194)
Cash flows from investing activities:
Purchase of short-term investments	(32,982,710)	(57,121,593)
Proceeds from the sale of short-term investments	63,581,849	33,584,980
Purchases of equipment and furnishings	(325,471)	(590,077)
Net cash provided by (used in) investing activities	30,273,668	(24,126,690)
Cash flows from financing activities:
Net proceeds from public offering	26,780,068	80,535,401
Proceeds from issuance of restricted stock to employee	0	100
Repurchase of common stock for treasury	0	(146,374)
Net proceeds from exercise of warrants and stock options	590,001	431,941
Net cash provided by financing activities	27,370,069	80,821,068
Net increase in cash and cash equivalents	23,560,715	27,929,184
Cash and cash equivalents at beginning of period	32,218,905	11,483,112
Cash and cash equivalents at end of period	55,779,620	39,412,296
Supplemental disclosure of cash flow information:
Equipment and furnishings purchased on credit	2,035	155,771
Cashless warrant exercises	3	133
Repurchase of Company's own stock for treasury	0	11,743
Cash paid for income taxes	$ 800	$ 77,071