Share-based compensation - Summary of Status of Company's Stock Option Plan (Detail) - Stock Option Plan - $ / shares	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Shares
Outstanding at beginning of period	3,069,440	3,214,398	2,633,589
-Exercised	(5,000)	(1,545,161)	(589,725)
-Granted	3,874,200	1,400,203	1,170,534
-Cancelled	(2,523,243)
Outstanding at end of period	4,415,397	3,069,440	3,214,398
Weighted Average Exercise Price
Outstanding at beginning of period	$ 4.28	$ 2.68	$ 2.06
-Exercised	3.43	2.27	2.43
-Granted	1.19	5.72	3.95
-Cancelled	4.92
Outstanding at end of period	$ 1.20	$ 4.28	$ 2.68

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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- Definition

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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