Quarterly Data (Unaudited) - Quarterly Results of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Data (Unaudited)
Total revenue	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
Operating expenses	3,727	3,615	5,157	7,046	5,988	6,701	6,996	7,126	19,535	$ 26,811	$ 29,721
Net loss	$ (3,810)	$ (3,792)	$ (5,344)	$ (6,833)	$ (6,240)	$ (7,102)	$ (7,446)	$ (7,516)	$ (19,779)	$ (28,304)	$ (28,741)
Basic and diluted net loss per common share (in dollars per share)	$ (0.11)	$ (0.11)	$ (0.16)	$ (0.20)	$ (0.18)	$ (0.22)	$ (0.26)	$ (0.26)	$ (0.58)	$ (0.91)	$ (1.02)