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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 |

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- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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