Selected Quarterly Financial Data (unaudited) (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Selected Quarterly Financial Information [Abstract]
Revenues	$ 12	$ 33	$ 13	$ 12	$ 12	$ 33	$ 13	$ 12	$ 70,000	$ 70,000	$ 70,353,483
Operating Expenses [Abstract]
Research and development	8,188	5,985	6,788	6,531	6,386	6,582	6,724	6,617	27,492,185	26,309,475	41,649,389
General and administrative	3,096	2,988	7,394	3,820	3,527	3,776	4,071	4,360	17,298,139	15,733,462	23,677,366
Loss from operations	(11,272)	(8,940)	(14,169)	(10,339)	(9,901)	(10,325)	(10,782)	(10,965)	(44,720,324)	(41,972,937)	5,026,728
Interest income, net	232	232	208	189	162	153	185	152	861,212	651,543	577,447
Other expense, net	(25)		(1)	(3)	(55)	170	(7)	(57)	(28,461)	51,645	(144,698)
Net income (loss)	$ (11,065)	$ (8,708)	$ (13,962)	$ (10,153)	$ (9,794)	$ (10,002)	$ (10,604)	$ (10,870)	$ (43,887,573)	$ (41,269,749)	$ 5,459,477
Net loss per share -basic and diluted (in dollars per share)	$ (0.26)	$ (0.21)	$ (0.33)	$ (0.24)	$ (0.23)	$ (0.24)	$ (0.25)	$ (0.26)
Weighted average common shares outstanding - basic and diluted (in shares)	42,289,453	42,263,311	42,178,537	42,109,828	41,984,763	41,974,518	41,939,052	41,857,905