Stock-Based Compensation - Summary of Stock Option Activity (Detail) - $ / shares	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares Outstanding, Beginning Balance	195,820
Number of Shares, Granted	8,100
Number of Shares, Cancelled/forfeited/expired	(1,839)
Number of Shares Outstanding, Ending Balance	202,081	195,820
Number of Shares, Vested and unvested expected to vest, Ending Balance	189,738
Weighted Average Exercise Price Per Share, Outstanding, Beginning Balance	$ 41.41
Weighted Average Exercise Price Per Share, Granted	1.03
Weighted Average Exercise Price Per Share, Cancelled/forfeited/expired	104.08
Weighted Average Exercise Price Per Share, Outstanding, Ending Balance	39.14	$ 41.41
Weighted Average Exercise Price Per Share, Vested and unvested expected to vest, Ending Balance	$ 41.48
Weighted Average Remaining Contractual Term in Years, Outstanding	9 years	9 years 2 months 12 days
Weighted Average Remaining Contractual Term in Years, Vested and unvested expected to vest	8 years 10 months 24 days

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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.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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Gross number of share options (or share units) granted during the period.

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

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- Definition

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

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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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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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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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Weighted average remaining contractual term for fully vested and expected to vest options outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. 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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