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Summary of Option Activity Under Company's Equity Incentive Plans (Detail) - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Number of Shares Underlying Outstanding Options
Number of Shares Underlying Outstanding Options, Beginning Balance	1,361,977
Number of Shares Underlying Outstanding Options, Options granted	309,300
Number of Shares Underlying Outstanding Options, Options exercised	(12,794)
Number of Shares Underlying Outstanding Options, Options forfeited	(39,971)
Number of Shares Underlying Outstanding Options, Options Expired	(5,641)
Number of Shares Underlying Outstanding Options, Ending Balance	1,612,871	1,361,977
Number of Shares Underlying Outstanding Options, Exercisable - March 31, 2019	468,193
Number of Shares Underlying Outstanding Options, Vested and expected to vest - March 31, 2019	1,612,871
Weighted- Average Exercise Price
Weighted- Average Exercise Price, Beginning Balance	$ 12.04
Weighted- Average Exercise Price, Options granted	8.12
Weighted- Average Exercise Price, Options exercised	4.60
Weighted- Average Exercise Price, Options forfeited	7.60
Weighted- Average Exercise Price, Options Expired	91.79
Weighted- Average Exercise Price, Ending Balance	11.17	$ 12.04
Weighted- Average Exercise Price, Exercisable - March 31, 2019	16.19
Weighted- Average Exercise Price, Vested and expected to vest - March 31, 2019	$ 11.17
Weighted-Average Remaining Contractual Term (Years)
Weighted-Average Remaining Contractual Term (Years), Outstanding Balance	8 years 9 months 7 days	8 years 8 months 15 days
Weighted-Average Remaining Contractual Term (Years), Exercisable - March 31, 2019	8 years 10 days
Weighted-Average Remaining Contractual Term (Years), Vested and expected to vest - March 31, 2019	8 years 9 months 7 days

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Number of fully vested and expected to vest exercisable options that may be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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