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Summary of Significant Accounting Policies - New Accounting Policies (Details) - CAD CAD in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2016 | Dec. 31, 2015
New Accounting Pronouncement, Early Adoption [Line Items] |  |
Adoption of new accounting policy (Note 3) |  | CAD 16
Retained Earnings |  |
New Accounting Pronouncement, Early Adoption [Line Items] |  |
Adoption of new accounting policy (Note 3) |  | 16
Early adoption effect | Accounting Standards Update 2016-09 | Income tax expense |  |
New Accounting Pronouncement, Early Adoption [Line Items] |  |
Income tax reconciliation, share-based compensation excess tax benefit | CAD 7 |
Early adoption effect | Accounting Standards Update 2016-09 | Deferred income taxes |  |
New Accounting Pronouncement, Early Adoption [Line Items] |  |
Adoption of new accounting policy (Note 3) |  | (16)
Income tax reconciliation, share-based compensation excess tax benefit | CAD (7) |
Early adoption effect | Accounting Standards Update 2016-09 | Retained Earnings |  |
New Accounting Pronouncement, Early Adoption [Line Items] |  |
Adoption of new accounting policy (Note 3) |  | CAD 16
Waneta Partnership |  |
New Accounting Pronouncement, Early Adoption [Line Items] |  |
Controlling ownership interest (percent) | 51.00% |
Waneta Partnership | Variable Interest Entity |  |
New Accounting Pronouncement, Early Adoption [Line Items] |  |
Controlling ownership interest (percent) | 51.00% |

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