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
Quarterly Financial Data [Abstract]
Revenues	$ 68,000	$ 89,000	$ 104,000	$ 120,000	$ 79,000	$ 58,000	$ 49,000	$ 127,000	$ 381,072	$ 312,812	$ 280,178
Operating expenses	9,850,000	10,013,000	10,084,000	10,579,000	7,463,000	6,467,000	6,682,000	4,975,000	40,526,683	25,586,464	15,225,337
Net loss attributable to common stockholders	$ (8,554,000)	$ (10,197,000)	$ (10,208,000)	$ (10,269,000)	$ (7,387,000)	$ (2,742,000)	$ (10,186,000)	$ (7,180,000)	$ (39,227,959)	$ (27,495,334)	$ (14,348,499)
Net loss per common share - basic (in dollars per share)	$ (0.28)	$ (0.34)	$ (0.34)	$ (0.35)	$ (0.25)	$ (0.10)	$ (0.41)	$ (0.33)	$ (1.30)	$ (1.05)	$ (0.76)
Net loss per common share - diluted (in dollars per share)	$ (0.34)	$ (0.34)	$ (0.34)	$ (0.36)	$ (0.26)	$ (0.23)	$ (0.41)	$ (0.33)	$ (1.37)	$ (1.21)	$ (0.88)
Shares used in the calculation of net loss attributable to common stockholders - basic (in shares)	30,639,440	30,339,774	29,958,037	29,755,184	29,723,254	28,560,211	24,592,883	21,817,710	30,174,838	26,201,713	18,904,280
Shares used in the calculation of net loss attributable to common stockholders - diluted (in shares)	30,711,946	30,339,774	29,958,037	30,108,377	30,157,038	29,128,235	24,592,883	21,817,710	30,281,263	26,452,165	19,071,112