CONDENSED STATEMENTS OF CASH FLOWS - USD ($)	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Operating activities
Net income (loss)	$ (46,500,131)	$ (32,822,796)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	701,133	679,675
Stock-based compensation	2,680,873	7,889,999
Acquired in-process research and development	16,493,132
Loss on disposal of property and equipment	114,042
Accretion of bond premium	1,399	331,881
Change in operating assets and liabilities:
Receivables	157,805	170,934
Prepaid expenses and other assets	770,892	(490,175)
Accounts payable	(1,160,801)	104,079
Accrued wages, benefits and other liabilities	(633,288)	(1,933,414)
Deferred revenue	(37,500)	(37,500)
Net cash used in operating activities	(27,412,444)	(26,107,317)
Investing activities
Purchases of property and equipment	(46,833)	(664,673)
Purchases of investments	(51,318,003)	(114,900,851)
Purchase of acquired in-process research and development	(12,322,349)
Proceeds from sale and maturities of investments	102,430,000	160,865,053
Net cash provided by investing activities	38,742,815	45,299,529
Financing activities
Stock repurchase	(94,627)	(158,283)
Proceeds from exercise of warrant to purchase common stock	4,556,420
Proceeds from the exercise of stock options	109,742	129,306
Proceeds from stock purchases under employee stock purchase plan	48,407	137,925
Net cash provided by financing activities	4,619,942	108,948
Net increase (decrease) in cash and cash equivalents	15,950,313	19,301,160
Cash and cash equivalents at beginning of period	31,228,192	15,431,622
Cash and cash equivalents at end of period	$ 47,178,505	$ 34,732,782