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Income taxes (Narrative) (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Major Components Of Tax Expense (Income) [Line Items]    
Statutory tax rate 26.60% 26.70%
Unrecognized deferred tax liabilities $ 39.1 $ 76.8
Quebec [Member]    
Major Components Of Tax Expense (Income) [Line Items]    
Tax effect from change in tax rate $ 0.1 $ 0.3
Top of range [member] | Quebec [Member]    
Major Components Of Tax Expense (Income) [Line Items]    
Tax rate effect from change in tax rate 11.60% 11.70%
Bottom of range [member] | Quebec [Member]    
Major Components Of Tax Expense (Income) [Line Items]    
Tax rate effect from change in tax rate 11.50% 11.50%