Net Loss Per Share - Computation of Basic and Diluted Loss Per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net loss	$ (19,790)	$ (23,539)	$ (25,467)	$ (26,144)	$ (27,862)	$ (24,126)	$ (22,291)	$ (17,019)	$ (94,940)	$ (91,298)	$ (64,339)
Denominator:
Weighted average common shares outstanding - basic and diluted									64,478,066	56,219,919	47,435,331
Net loss per share - basic and diluted	$ (0.27)	$ (0.37)	$ (0.41)	$ (0.42)	$ (0.45)	$ (0.39)	$ (0.42)	$ (0.36)	$ (1.47)	$ (1.62)	$ (1.36)