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Income Taxes (Carry Overs) (Details) - CAD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Tax Loss Carry Forwards [Abstract]    
Deferred Tax Assets, Tax Credit Carryforwards $ 269 $ 314
Tax Loss Carry Forwards Valuation Allowance [Abstract]    
Deferred Tax Assets, Valuation Allowance 163 105
Capital Loss Carryforward [Member] | Canada Revenue Agency [Member]    
Tax Credit Carryforward [Line Items]    
Gross Tax Carryforwards, Tax credit / capital loss 86  
Unrecognized amounts, Tax credit / capital loss (77)  
Tax credit carryforwards, NOL / capital loss 9  
Capital Loss Carryforward [Member] | US Tax authority [Member]    
Tax Credit Carryforward [Line Items]    
Gross Tax Carryforwards, Tax credit / capital loss 6  
Unrecognized amounts, Tax credit / capital loss (6)  
Tax credit carryforwards, NOL / capital loss $ 0  
Capital Loss Carryforward [Member] | Minimum [Member] | US Tax authority [Member]    
Tax Credit Carryforward [Line Items]    
Expiration period, Tax credit / capital loss 2019  
Operating Loss Carryforward [Member]    
Tax Credit Carryforward [Line Items]    
Gross Tax Carryforwards, Tax credit / capital loss $ 34  
Unrecognized amounts, Tax credit / capital loss (34)  
Tax credit carryforwards, NOL / capital loss 0  
Operating Loss Carryforward [Member] | Canada Revenue Agency [Member]    
Tax Credit Carryforward [Line Items]    
Gross Tax Carryforwards, Tax credit / capital loss 817  
Unrecognized amounts, Tax credit / capital loss (405)  
Tax credit carryforwards, NOL / capital loss 412  
Operating Loss Carryforward [Member] | US Tax authority [Member]    
Tax Credit Carryforward [Line Items]    
Gross Tax Carryforwards, Tax credit / capital loss 2,848  
Unrecognized amounts, Tax credit / capital loss 0  
Tax credit carryforwards, NOL / capital loss $ 2,848  
Operating Loss Carryforward [Member] | Maximum [Member]    
Tax Credit Carryforward [Line Items]    
Expiration period, Tax credit / capital loss 2025  
Operating Loss Carryforward [Member] | Maximum [Member] | Canada Revenue Agency [Member]    
Tax Credit Carryforward [Line Items]    
Expiration period, Tax credit / capital loss 2038  
Operating Loss Carryforward [Member] | Maximum [Member] | US Tax authority [Member]    
Tax Credit Carryforward [Line Items]    
Expiration period, Tax credit / capital loss 2037  
Operating Loss Carryforward [Member] | Minimum [Member]    
Tax Credit Carryforward [Line Items]    
Expiration period, Tax credit / capital loss 2019  
Operating Loss Carryforward [Member] | Minimum [Member] | Canada Revenue Agency [Member]    
Tax Credit Carryforward [Line Items]    
Expiration period, Tax credit / capital loss 2027  
Operating Loss Carryforward [Member] | Minimum [Member] | US Tax authority [Member]    
Tax Credit Carryforward [Line Items]    
Expiration period, Tax credit / capital loss 2024  
State Operating Loss Carryforward [Member] | US Tax authority [Member]    
Tax Credit Carryforward [Line Items]    
Gross Tax Carryforwards, Tax credit / capital loss $ 1,314  
Unrecognized amounts, Tax credit / capital loss (47)  
Tax credit carryforwards, NOL / capital loss $ 1,267  
State Operating Loss Carryforward [Member] | Maximum [Member] | US Tax authority [Member]    
Tax Credit Carryforward [Line Items]    
Expiration period, Tax credit / capital loss 2038  
State Operating Loss Carryforward [Member] | Minimum [Member] | US Tax authority [Member]    
Tax Credit Carryforward [Line Items]    
Expiration period, Tax credit / capital loss 2024  
Tax credit carry forwards [Member] | US Tax authority [Member]    
Tax Credit Carryforward [Line Items]    
Gross Tax Carryforwards, Tax credit / capital loss $ 268  
Unrecognized amounts, Tax credit / capital loss 0  
Tax credit carryforwards, NOL / capital loss $ 268  
Tax credit carry forwards [Member] | Minimum [Member] | US Tax authority [Member]    
Tax Credit Carryforward [Line Items]    
Expiration period, Tax credit / capital loss 2019  
Domestic Tax Authority [Member]    
Tax Credit Carryforward [Line Items]    
Gross tax carry forwards, NOL $ 33 40
Foreign Tax Authority [Member]    
Tax Credit Carryforward [Line Items]    
Gross tax carry forwards, NOL 0 194
Other Tax Authority [Member]    
Tax Credit Carryforward [Line Items]    
Gross tax carry forwards, NOL $ 1 $ 0