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Income Taxes - Narratives (Details) - CAD ($)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Provisions [Line Items]    
Corresponding increase in tax expense $ 20,785,000 $ 19,906,000
Letters of credit oustanding 1,572,984,000 1,474,155,000
Canadian Revenue Agency [member]    
Provisions [Line Items]    
Amount of potential taxable income consequences from tax dispute $ 4,900,000,000  
Percentage of taxes and interest charges 50.00%  
Penalties related to transfer pricing [member] | Canadian Revenue Agency [member]    
Provisions [Line Items]    
Provision for CRA transfer price dispute $ 371,000,000  
Provision for uncertain tax positions [member] | Canadian Revenue Agency [member]    
Provisions [Line Items]    
Corresponding increase in tax expense 2,600,000,000  
Receivables from tax authorities 303,222,000 $ 303,222,000
Letters of credit oustanding $ 478,000,000  
Percentage of taxes and interest charges 50.00%  
Provisions $ (61,000,000)  
Provision for uncertain tax positions [member] | Canadian Revenue Agency [member] | Bottom of range [member]    
Provisions [Line Items]    
Amount of potential taxable income consequences from tax dispute 4,900,000,000  
Provision for uncertain tax positions [member] | Canadian Revenue Agency [member] | Top of range [member]    
Provisions [Line Items]    
Amount of potential taxable income consequences from tax dispute 8,700,000,000  
Provision for uncertain tax positions [member] | Penalties related to cash taxes and transfer pricing [member] | Canadian Revenue Agency [member] | Bottom of range [member]    
Provisions [Line Items]    
Provision for CRA transfer price dispute 1,950,000,000  
Provision for uncertain tax positions [member] | Penalties related to cash taxes and transfer pricing [member] | Canadian Revenue Agency [member] | Top of range [member]    
Provisions [Line Items]    
Provision for CRA transfer price dispute 2,150,000,000  
Provision for uncertain tax positions [member] | 50% Penalties related to cash tax and transfer pricing [member] | Canadian Revenue Agency [member] | Bottom of range [member]    
Provisions [Line Items]    
Provision for CRA transfer price dispute 970,000,000  
Provision for uncertain tax positions [member] | 50% Penalties related to cash tax and transfer pricing [member] | Canadian Revenue Agency [member] | Top of range [member]    
Provisions [Line Items]    
Provision for CRA transfer price dispute $ 1,070,000,000