Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	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 Information (Unaudited)
Operating expenses	$ 8,673	$ 8,283	$ 9,396	$ 9,206	$ 9,333	$ 8,201	$ 7,765	$ 6,360	$ 35,558	$ 31,659	$ 21,681
Net loss	$ (8,778)	$ (8,419)	$ (9,445)	$ (9,418)	$ (9,606)	$ (8,485)	$ (8,039)	$ (6,500)	$ (35,558)	$ (31,659)	$ (21,681)
Basic and Diluted (in dollars per share)	$ (0.47)	$ (0.54)	$ (0.61)	$ (0.61)	$ (0.63)	$ (0.55)	$ (8.07)	$ (13.14)	$ (2.22)	$ (4.06)	$ (51.56)
Basic and Diluted (in shares)	18,699,480	15,512,608	15,373,964	15,335,516	15,298,810	15,297,794	996,592	494,590	16,230,190	8,041,948	424,477