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Stock-based Compensation (Narrative) (Details) - CAD
3 Months Ended 12 Months Ended
Dec. 31, 2016
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock Issued During Period, Shares, Conversion of Units   51,990,000 0
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 4,000,000  
Allocated Share-based Compensation Expense   CAD 1,000,000 CAD 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   CAD 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   CAD 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 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   CAD 2,000,000 1,000,000
Share Based Compensation Arrangement By Share Based Payment Award Options Exercises In Period Total Cash Received   16,000,000 2,000,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value   CAD 13,000,000 CAD 1,000,000
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Lower Range Limit   CAD 20.42 CAD 19.88
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range Upper Range Limit   CAD 46.19 CAD 42.71
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   CAD 8,000,000 CAD 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   CAD 3,000,000 3,000,000
Deferred Tax Liabilities, Regulatory Assets and Liabilities CAD 0 CAD 0 1,000,000
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   CAD 11,000,000 10,000,000
Number Of Trading Days   50 days  
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense   CAD 4,000,000 CAD 3,000,000
Performance-based Share Unit Plan Performance Cycle Term   3 years