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Share-based compensation plans - Inputs used in the measurement (Details)
12 Months Ended
Dec. 31, 2018
CAD ($)
shares
yr
Dec. 31, 2017
CAD ($)
shares
Dec. 31, 2016
shares
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Number of options granted | shares 1,473,430 1,373,040  
Average strike price $ 11.32 $ 14.7  
Number of options, reporting date | shares 8,820,805 8,324,666 8,020,311
Average strike price, reporting date $ 22.83 $ 24.95  
Stock Option Plan [member]      
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Number of options granted | shares 1,473,430    
Average strike price $ 11.32    
Expected dividend $ 0.08    
Expected volatility 35.00%    
Risk-free interest rate 2.00%    
Expected life of option | yr 4.8    
Expected forfeitures 7.00%    
Weighted average grant date fair values $ 3.48    
Performance Share Unit Plan [member]      
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Number of options granted | shares 602,530    
Average strike price $ 0    
Expected dividend $ 0    
Expected volatility 37.00%    
Risk-free interest rate 1.90%    
Expected life of option | yr 3    
Expected forfeitures 9.00%    
Weighted average grant date fair values $ 11.43    
Restricted Share Unit Plan [member]      
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Number of options granted | shares 377,021    
Average strike price $ 11.46    
Expected dividend $ 0    
Expected volatility 0.00%    
Risk-free interest rate 0.00%    
Expected life of option | yr 0    
Expected forfeitures 13.00%    
Weighted average grant date fair values $ 11.46    
Phantom Stock Option Plan [member]      
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Number of options granted | shares 101,895    
Average strike price $ 11.32    
Expected dividend $ 0.08    
Expected volatility 35.00%    
Risk-free interest rate 1.90%    
Expected life of option | yr 4.5    
Expected forfeitures 7.00%    
Weighted average grant date fair values $ 3.68    
Number of options, reporting date | shares 353,580    
Average strike price, reporting date $ 17.74 $ 22.13  
Expected dividend, reporting date $ 0.08    
Expected volatility, reporting date 37.00%    
Risk free interest rate, reporting date 1.90%    
Expected life of option, reporting date | yr 3.7    
Expected forfeitures, reporting date 8.00%    
Weighted average share price, reporting date $ 4.21