Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information (unaudited)
Research and development expenses	$ 6,061	$ 3,472	$ 3,915	$ 5,468	$ 2,167	$ 1,569	$ 2,805	$ 2,149	$ 18,916	$ 8,690
General and administrative expenses	1,436	1,378	1,255	1,447	1,387	868	458	517	5,516	3,230
Net loss					(3,607)	(1,668)	(3,323)	(2,235)	(24,850)	(10,833)
Cumulative preferred stock dividends						(372)	(1,102)	(1,071)		(2,545)
Net loss applicable to common stockholders	$ (7,607)	$ (4,963)	$ (5,273)	$ (7,007)	$ (3,607)	$ (2,040)	$ (4,425)	$ (3,306)	$ (24,850)	$ (13,378)
Net loss per share, basic and diluted	$ (0.45)	$ (0.35)	$ (0.37)	$ (0.50)	$ (0.26)	$ (0.22)	$ (7.98)	$ (7.09)	$ (1.67)	$ (2.17)