Stock-Based Compensation - Summary of Stock Option Activity Under the 2012 and 2013 Plans (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Number of Options, Outstanding Beginning Balance	3,339,471
Number of Options, Cancelled	(1,536,304)
Number of Options, Outstanding Ending Balance	1,803,167	3,339,471
Number of Options, Vested and expected to vest outstanding	1,766,323
Number of Options, Exercisable	1,723,797
Weighted Average Exercise Price, Outstanding Beginning Balance	$ 1.03
Weighted Average Exercise Price, Cancelled	0.57
Weighted Average Exercise Price, Outstanding Ending Balance	1.03
Weighted Average Exercise Price, Vested and expected to vest outstanding	1.41
Weighted Average Exercise Price, Exercisable	$ 1.37
Weighted Average Remaining Contractual Term, Outstanding	7 years 2 months 12 days	8 years 1 month 6 days
Weighted Average Remaining Contractual Term, Vested and expected to vest outstanding	7 years 2 months 12 days
Weighted Average Remaining Contractual Term, Exercisable	7 years 1 month 6 days
Aggregate Intrinsic Value, Outstanding	$ 0	$ 0
Aggregate Intrinsic Value ,Vested and expected to vest outstanding	0
Aggregate Intrinsic Value, Exercisable	$ 0

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest options outstanding. 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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Number of fully vested and expected to vest options outstanding that can 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 exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest options outstanding. 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 grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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