Share-Based Payments - Summary of Stock Option Activity (Detail)	6 Months Ended
Jun. 30, 2018 $ / shares shares
Shares
Outstanding at beginning of period (in shares) \| shares	2,695,796
Granted (in shares) \| shares	1,643,400
Exercised (in shares) \| shares	(55,259)
Cancelled or forfeited (in shares) \| shares	(256,111)
Outstanding at end of period (in shares) \| shares	4,027,826
Exercisable at end of period (in shares) \| shares	1,738,331
Vested and expected to vest at end of period (in shares) \| shares	3,977,226
Weighted-Average Exercise Price
Outstanding at beginning of period (in USD per share) \| $ / shares	$ 3.16
Granted (in USD per share) \| $ / shares	1.82
Exercised (in USD per share) \| $ / shares	0.53
Cancelled or forfeited (in USD per share) \| $ / shares	2.95
Outstanding at end of period (in USD per share) \| $ / shares	2.66
Exercisable at end of period (in USD per share) \| $ / shares	3.53
Vested and expected to vest at end of period (in USD per share) \| $ / shares	$ 2.67

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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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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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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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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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 option holders acquired shares when converting their stock options into shares.

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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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Number of share options (or share units) exercised during the current period.

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