CONDENSED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($)	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Cash flows from operating activities:
Net loss	$ (43,098,785)	$ (36,284,995)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	364,548	240,391
Stock-based compensation expense	5,400,604	4,520,307
Fair value adjustment on warrant liability	(939,668)	(4,079,748)
Amortization of loan cost and discount	382,241	0
Loss on retirement of fixed assets	9,116	0
Changes in assets and liabilities:
Receivables	4,485,130	351,388
Interest receivable	28,130	97,873
Prepaid expenses and other assets	781,013	1,545,955
Accounts payable	(3,254,181)	(447,430)
Accrued expenses and other current liabilities	(4,644,353)	(26,763)
Net cash used in operating activities	(40,486,205)	(34,083,022)
Cash flows from investing activities:
Purchase of short-term investments	0	(32,982,710)
Proceeds from the sale of short-term investments	35,035,420	63,581,849
Purchases of equipment and furnishings	(961,221)	(325,471)
Net cash provided by investing activities	34,074,199	30,273,668
Cash flows from financing activities:
Net proceeds from public offering	18,309,781	26,780,068
Net proceeds from term loan	24,012,078	0
Net proceeds from exercise of warrants and stock options	704,700	590,001
Net cash provided by financing activities	43,026,559	27,370,069
Net increase in cash and cash equivalents	36,614,553	23,560,715
Cash and cash equivalents at beginning of period	22,261,372	32,218,905
Cash and cash equivalents at end of period	58,875,925	55,779,620
Supplemental disclosure of cash flow information:
Cash paid during the year for interest	1,359,028	0
Cashless warrant exercises	0	3
Cash paid for income taxes	800	800
Supplemental disclosure of non-cash activities:
Warrants issued in connection with term loan	633,749	0
Equipment and furnishings purchased on credit	11,154	2,035
Shares issued in connection with the class action settlement	4,500,000	0
Warrants issued in connection with public offering	$ 6,932,592	$ 0