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Stock-based Compensation (Narrative) (Details) - CAD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock Issued During Period, Shares, Conversion of Units 150,000 51,990,000
Employee Common Share Purchase Plan [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized 4,000,000  
Allocated Share-based Compensation Expense $ 1,000,000 $ 1,000,000
Employee Common Share Purchase Plan [Member] | Scenario 1 [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Employee Stock Purchase Plan (ESSP), Employer Matching Percentage 20.00%  
Employee Common Share Purchase Plan [Member] | Scenario 2 [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Employee Stock Purchase Plan (ESSP), Employer Matching Percentage 10.00%  
Employee Common Share Purchase Plan [Member] | Minimum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Employee Stock Purchase Plan (ESSP), Employee Contribution $ 25  
Employee Common Share Purchase Plan [Member] | Minimum [Member] | Scenario 2 [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Employee Stock Purchase Plan (ESSP), Employee Contribution 3,000  
Employee Common Share Purchase Plan [Member] | Maximum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Employee Stock Purchase Plan (ESSP), Employee Contribution 8,000  
Employee Common Share Purchase Plan [Member] | Maximum [Member] | Scenario 1 [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Employee Stock Purchase Plan (ESSP), Employee Contribution 3,000  
Employee Common Share Purchase Plan [Member] | Maximum [Member] | Scenario 2 [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Employee Stock Purchase Plan (ESSP), Employee Contribution $ 8,000  
Common Shareholders Dividend Reinvestment and Share Purchase Plan [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Dividend Reinvestment Plan Percentage Discount From Average Market Price Of Common Shares 5.00%  
Stock Option Plan [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized 11,700,000  
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 10 years  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Exercisable Options, Contractual Term, Employee Retirement Or Termination For Other Than Just Cause 24 months  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Exercisable Options, Contractual Term, Employee Termination For Just Cause Or Resignation Or Death 6 months  
Fair Value Assumptions, Historical Volatility Period 5 years  
Employee Service Share-based Compensation, Allocation of Recognized Period Costs, Capitalized Amount $ 2,000,000 2,000,000
Share Based Compensation Arrangement By Share Based Payment Award Options Exercises In Period Total Cash Received 3,000,000 16,000,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value $ 2,000,000 $ 13,000,000
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Lower Range Limit $ 21.58 $ 20.42
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range Upper Range Limit $ 46.19 $ 46.19
Stock Option Plan [Member] | First Anniversary [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage 25.00%  
Stock Option Plan [Member] | Second Anniversary [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage 25.00%  
Stock Option Plan [Member] | Third Anniversary [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage 25.00%  
Stock Option Plan [Member] | Fourth Anniversary [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage 25.00%  
Deferred Share Unit Plans [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Employee Service Share-based Compensation, Allocation of Recognized Period Costs, Capitalized Amount $ 7,000,000 $ 8,000,000
Stock Issued During Period, Shares, Conversion of Units 1  
Number Of Trading Days 10 days  
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense $ 2,000,000 3,000,000
Deferred Tax Liabilities, Regulatory Assets and Liabilities $ 0  
Deferred Share Unit Plans [Member] | Scenario 1 [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Deferred Share Unit Plan Percentage Of Value Of Actual Annual Incentive Award 25.00%  
Deferred Share Unit Plans [Member] | Scenario 2 [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Deferred Share Unit Plan Percentage Of Value Of Actual Annual Incentive Award 50.00%  
Performance Share Unit Plan [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Employee Service Share-based Compensation, Allocation of Recognized Period Costs, Capitalized Amount $ 14,000,000 11,000,000
Number Of Trading Days 50 days  
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense $ 4,000,000 $ 4,000,000
Performance-based Share Unit Plan Performance Cycle Term 3 years