Selected Quarterly Consolidated Financial Data (Unaudited) - Quarterly Consolidated Financial Information for Interim Periods (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 16	$ 264	$ 15	$ 16	$ 376	$ 413	$ 413	$ 601	$ 311	$ 1,803	$ 15,577
Loss from operations	(8,051)	(8,644)	(7,893)	(7,987)	(5,462)	(7,331)	(6,366)	(5,658)	(32,575)	(24,817)	(9,361)
Net loss	$ (8,065)	$ (7,742)	$ (7,380)	$ (7,517)	$ (6,004)	$ (7,714)	$ (6,016)	$ (3,263)	$ (30,704)	$ (22,997)	$ (15,752)
Net loss per common share - basic	$ (0.45)	$ (0.43)	$ (0.41)	$ (0.42)					$ (1.71)	$ (1.48)	$ (1.10)
Net loss per common share - diluted	$ (0.45)	$ (0.43)	$ (0.41)	$ (0.43)					$ (1.72)	$ (1.48)	$ (1.10)
Basic and diluted net loss per common share					$ (0.34)	$ (0.51)	$ (0.42)	$ (0.23)