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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator:
Net loss attributable to common stockholders	$ (7,917)	$ (8,755)	$ (8,985)	$ (5,896)	$ (12,736)	$ (9,865)	$ (9,211)	$ (9,195)	$ (31,553)	$ (41,007)	$ (32,872)
Denominator:
Weighted-average common shares outstanding - basic and diluted									19,595	12,653	12,080
Net loss per common share - basic and diluted	$ (0.21)	$ (0.63)	$ (0.69)	$ (0.45)	$ (0.99)	$ (0.77)	$ (0.74)	$ (0.74)	$ (1.61)	$ (3.24)	$ (2.72)