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Share capital - Summary of Share Capital (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure of classes of share capital [line items]    
Shares issued upon exercise of share options, for cash (in shares)   48,529
Shares issued for acquisition of Integra (in shares)   15,436,179
Balance beginning of year $ 3,723,482  
Shares issued upon exercise of share options, for cash   $ 586
Estimated fair value of share options exercised   176
Balance end of year $ 3,347,086 $ 3,723,482
Voting common shares [member]    
Disclosure of classes of share capital [line items]    
Beginning balance (in shares) 158,801,722 143,317,014
Ending balance (in shares) 158,801,722 158,801,722
Share capital [member]    
Disclosure of classes of share capital [line items]    
Balance beginning of year $ 3,007,924 $ 2,819,101
Shares issued upon exercise of share options, for cash 0 586
Shares issued for acquisition of Integra   188,061
Balance end of year $ 3,007,924 $ 3,007,924