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Basic and diluted net income / (loss) per share (Schedule of Anti-dilutive Shares Excluded from Computation of Diluted Net Income per Common Share) (Detail) - shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Stock options and restricted shares [Member]      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Share options and restricted shares —weighted average 5,621,418 2,902,950 1,673,342