Quarterly Financial Information (unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information (unaudited)
Research and development expenses	$ 2,167	$ 1,569	$ 2,805	$ 2,149	$ 1,068	$ 1,008	$ 1,327	$ 747	$ 8,690	$ 4,150
General and administrative expenses	1,387	868	458	517	422	313	314	180	3,230	1,229
Net loss	(3,607)	(1,668)	(3,323)	(2,235)	(1,394)	(1,224)	(1,682)	(970)	(10,833)	(5,270)
Cumulative preferred stock dividends		(372)	(1,102)	(1,071)	(1,049)	(1,029)	(940)	(786)	(2,545)	(3,804)
Net loss applicable to common shareholders	$ (3,607)	$ (2,040)	$ (4,425)	$ (3,306)	$ (2,443)	$ (2,253)	$ (2,622)	$ (1,756)	$ (13,378)	$ (9,074)
Net loss per share, basic and diluted (in dollars per share)	$ (0.26)	$ (0.22)	$ (7.98)	$ (7.09)	$ (5.54)	$ (4.58)	$ (5.66)	$ (3.82)	$ (2.17)	$ (19.60)