Net Loss Per Share Attributable to Common Stockholders - Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Numerator:
Net loss attributable to common stockholders	$ (6,890)	$ (5,171)	$ (7,013)	$ (6,547)	$ (7,917)	$ (8,755)	$ (8,985)	$ (5,896)	$ (25,621)	$ (31,553)	$ (41,007)
Denominator:
Weighted-average common shares outstanding - basic and diluted									39,754	19,595	12,653
Net loss per common share - basic and diluted	$ (0.17)	$ (0.13)	$ (0.18)	$ (0.17)	$ (0.21)	$ (0.63)	$ (0.69)	$ (0.45)	$ (0.64)	$ (1.61)	$ (3.24)