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Income Taxes - Deferred Income Tax Asset (Details) - CAD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax assets $ 100.0 $ 128.0 $ 100.0  
Capital losses on foreign exchange   1,134.0 556.0  
Deferred income tax liabilities 11,533.0 12,045.0 11,533.0  
Deferred income tax liability (asset) 11,433.0 $ 11,917.0 $ 11,433.0 $ 11,180.0
Tax rate   27.04% 27.01%  
Tax loss carryforwards        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax assets 75.0 $ 153.0 $ 75.0  
Earnings retained in our investments in foreign subsidiaries        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Deferred income tax liabilities $ 9,600.0 $ 9,700.0 $ 9,600.0  
United States Government        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Tax rate   35.00%    
Deferred income tax expense (recovery)   $ (124.0)    
United States Government | Scenario, Forecast [Member]        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Tax rate   21.00%    
Government of British Columbia        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Tax rate 11.00%      
Enacted tax rate 12.00%      
Deferred income tax expense (recovery) $ 18.0