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Asset Impairment Charges and Reversals (Details) - CAD |  |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jan. 26, 2017 | Nov. 24, 2016 | Jun. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Canadian Coal |  |  |  |  |  |
Disclosure of impairment loss recognised or reversed [line items] |  |  |  |  |  |
Annual cash payments |  | CAD 37,400,000 |  |  |  |
Wintering Hills |  |  |  |  |  |
Disclosure of impairment loss recognised or reversed [line items] |  |  |  |  |  |
Equity participation percentage | 51.00% |  |  |  |  |
Cash transferred | CAD 61,000,000 |  |  |  |  |
Sundance Unit 1 |  |  |  |  |  |
Disclosure of impairment loss recognised or reversed [line items] |  |  |  |  |  |
Impairment loss |  |  | CAD 20,000,000 |  |  |
Centralia Gas |  |  |  |  |  |
Disclosure of impairment loss recognised or reversed [line items] |  |  |  |  |  |
Reversal of impairment loss |  |  |  |  |  | CAD 2,000,000
Investments accounted for using equity method | Alberta Merchant CGU |  |  |  |  |  |
Disclosure of impairment loss recognised or reversed [line items] |  |  |  |  |  |
Levy on carbon emissions in excess of defined limits |  |  |  | CAD 20 | CAD 30 |
Minimum | Investments accounted for using equity method | Alberta Merchant CGU |  |  |  |  |  |
Disclosure of impairment loss recognised or reversed [line items] |  |  |  |  |  |
Price on geenhouse gas emissions |  |  |  |  |  | 10
Maximum | Investments accounted for using equity method | Alberta Merchant CGU |  |  |  |  |  |
Disclosure of impairment loss recognised or reversed [line items] |  |  |  |  |  |
Price on geenhouse gas emissions |  |  |  |  |  | CAD 50

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