Share-based compensation - Summary of Status of Company's Stock Option Plan (Detail) - Stock Option Plan - $ / shares	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Shares
Outstanding at beginning of period	3,069,440	3,214,398
-Exercised	(5,000)	(1,545,161)
-Granted	231,500	1,400,203
-Cancelled	(2,510,243)
Outstanding at end of period	785,697	3,069,440
Weighted Average Exercise Price
Outstanding at beginning of period	$ 4.28	$ 2.68
-Exercised	3.43	2.27
-Granted	2.54	5.72
-Cancelled	4.94
Outstanding at end of period	$ 1.65	$ 4.28

No definition available.

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- Definition

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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Net number of share options (or share units) granted during the period.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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- Definition

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

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-Publisher FASB
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- Definition

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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- Definition

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

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- Definition

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

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- Definition

Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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

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