Net Income Per Share - Computation of Basic and Diluted Net Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Basic:
Net income	$ 473	$ 1,039
Net income allocable to preferred stockholders		768
Net income allocable to common stockholders	473	271
Diluted:
Net income	473	1,039
Net income allocable to preferred stockholders		725
Net income allocable to common stockholders	$ 473	$ 314
Denominator:
Weighted-average shares outstanding in computing basic net income per share allocable to common stockholders	16,823,851	3,785,022
Effect of potentially dilutive securities:
Stock options	1,353,754	858,954
Weighted-average shares used in computing diluted net income per common share allocable to common stockholders	18,177,605	4,643,976
Net income per common share:
Basic	$ 0.03	$ 0.07
Diluted	$ 0.03	$ 0.07