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Summary of Significant Accounting Policies - Income Taxes, Sales Taxes, and New Accounting Policies (Details) - CAD CAD in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2016 | Dec. 31, 2015
Variable Interest Entity [Line Items] |  |
Undistributed earnings on foreign subsidiaries | CAD 525 | CAD 565
Hydroelectric Power Generation | BECOL |  |
Variable Interest Entity [Line Items] |  |
Long-term contract for electric power, term | 50 years |

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