Shareholders' Equity - Summary of Stock Option Activity (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Number of Shares, Beginning Balance	1,846,768	1,778,697
Granted, Number of Shares	248,100	903,248
Exercised, Number of Shares	(54,692)	(371,845)
Forfeited, Number of Shares	(91,411)	(396,161)
Expired, Number of Shares	(36,604)	(67,171)
Number of Shares, Ending Balance	1,912,161	1,846,768	1,778,697
Vested and expected to vest, Number of Shares, Ending Balance	1,792,667
Exercisable, Number of Shares, Ending Balance	1,065,767
Weighted Average Exercise Price, Beginning Balance	$ 4.84	$ 4.43
Granted, Weighted Average Exercise Price	5.18	5.24
Exercised, Weighted Average Exercise Price	3.66	3.17
Forfeited, Weighted Average Exercise Price	5.31	5.32
Expired, Weighted Average Exercise Price	5.83	5.91
Weighted Average Exercise Price, Ending Balance	4.88	$ 4.84	$ 4.43
Vested and expected to vest, Weighted Average Exercise Price, Ending Balance	4.85
Exercisable, Weighted Average Exercise Price, Ending Balance	$ 4.48
Balance outstanding, Weighted Average Remaining Contractual Life (in years)	7 years 7 months 9 days	8 years 2 months 8 days	7 years 9 months 29 days
Vested and expected to vest, Weighted Average Remaining Contractual Life (in years)	7 years 6 months 7 days
Exercisable, Weighted Average Remaining Contractual Life (in years)	6 years 7 months 17 days
Balance outstanding, Aggregate Intrinsic Value	$ 781,735
Vested and expected to vest, Aggregate Intrinsic Value	781,025
Exercisable, Aggregate Intrinsic Value	$ 775,767