Quarterly Data (Unaudited) - Quarterly Results of Operations (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	Dec. 31, 2013
Summary of quarterly results of operations
Total revenue									$ 0
Operating expenses	$ 9,165	$ 8,965	$ 7,982	$ 6,977	$ 6,524	$ 6,049	$ 3,495	$ 2,448	33,089	$ 18,515	$ 12,728
Net income (loss)	$ (3,899)	$ (9,411)	$ (8,486)	$ (8,538)	$ (6,845)	$ (6,353)	$ (3,718)	$ 839	$ (30,335)	$ (16,077)	$ (14,321)
Basic net income (loss) per common share (in dollars per share)	$ (0.17)	$ (0.42)	$ (0.38)	$ (0.40)	$ (0.37)	$ (0.34)	$ (0.46)	$ 0.10
Diluted net income (loss) per common share (in dollars per share)	$ (0.17)	$ (0.42)	$ (0.38)	$ (0.40)	$ (0.37)	$ (0.34)	$ (0.46)	$ 0.01