Selected Quarterly Financial Data (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Collaboration revenue	$ 3,315,457	$ 2,659,613	$ 2,359,311	$ 684,369					$ 9,018,750	$ 0	$ 0
Operating expenses	15,595,075	15,109,793	15,926,562	14,614,198	$ 11,868,254	$ 11,678,925	$ 10,454,130	$ 11,277,152	61,245,628	45,278,461	29,655,630
Interest and other income	365,541	241,326	239,858	136,484	225,681	378,381	444,605	490,421	983,209	1,539,088	1,228,830
Realized loss from marketable securities	(37,828)	(99,068)	(340,984)	(137,248)	(521,786)	(273,573)	(142,675)	(201,827)	(615,128)	(1,139,861)	(27,033)
Net loss	$ (3,007,072)	$ (12,272,019)	$ (13,598,864)	$ (13,930,593)	$ (11,546,687)	$ (11,574,117)	$ (10,152,200)	$ (10,988,558)	$ (42,808,548)	$ (44,261,562)	$ (28,453,833)
Basic and diluted net loss per common share	$ (0.16)	$ (0.71)	$ (0.78)	$ (0.81)	$ (0.67)	$ (0.67)	$ (0.59)	$ (0.64)	$ (2.41)	$ (2.57)	$ (1.81)