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Shareholders' equity - Outstanding (Detail)
Dec. 31, 2018
EquityInstruments
Dec. 31, 2018
shares
Dec. 31, 2018
USD ($)
Dec. 31, 2017
EquityInstruments
Dec. 31, 2017
shares
Dec. 31, 2017
USD ($)
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