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Shareholders' equity - Outstanding (Detail)
Dec. 31, 2018
EquityInstruments
Dec. 31, 2018
USD ($)
Dec. 31, 2018
shares
Dec. 31, 2017
EquityInstruments
Dec. 31, 2017
USD ($)
Dec. 31, 2017
shares
Dec. 31, 2016
EquityInstruments
Shareholders' equity              
Common share outstanding     186,692,481     185,930,331  
Stock options 1,015,409 1,015,409   955,603 955,603   1,304,328
Restricted Share Units | $   115,337     119,796    
Total     187,823,227     187,005,730