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Common shares - Additional Information (Detail) - $ / shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Common stock, shares authorized 1,000,000,000 1,000,000,000
Common stock, par value $ 0.00025 $ 0.00025
Common stock, shares outstanding 333,643,560 330,545,000
Common stock voting rights, description The holders of common shares are also entitled to receive dividends whenever funds are legally available and when declared by the Board of Directors, which is subject to the approval by the holders of the common shares representing a majority of the aggregate voting power of all outstanding shares.