CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenue:
Collaboration revenue	$ 70,000	$ 70,000	$ 70,353,483
Total revenue	70,000	70,000	70,353,483
Operating expenses:
Research and development	27,492,185	26,309,475	41,649,389
General and administrative	17,298,139	15,733,462	23,677,366
Total operating expenses	44,790,324	42,042,937	65,326,755
Loss from operations	(44,720,324)	(41,972,937)	5,026,728
Other income (expense), net:
Interest income, net	861,212	651,543	577,447
Other income (expense), net	(28,461)	51,645	(144,698)
Net income (loss)	$ (43,887,573)	$ (41,269,749)	$ 5,459,477
Net income (loss) per share - basic (in dollars per share)	$ (1.04)	$ (0.98)	$ 0.14
Net income (loss) per share - diluted (in dollars per share)	$ (1.04)	$ (0.98)	$ 0.13
Items included in other comprehensive income (loss):
Unrealized gain (loss) on foreign currency translation		$ 50,592	$ (38,776)
Unrealized gain (loss) and amounts reclassified to net loss on available-for-sale securities	$ 38,203	13,937	(161,843)
Other comprehensive income (loss)	38,203	64,529	(200,619)
Comprehensive income (loss)	$ (43,849,370)	$ (41,205,220)	$ 5,258,858
Weighted-average number of common shares used in net income (loss) per share calculation - basic (in shares)	42,210,643	41,939,504	40,242,352
Weighted-average number of common shares used in net income (loss) per share calculation - diluted (in shares)	42,210,643	41,939,504	41,758,101