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Asset Impairment Charges and Reversals Estimated The Fair Value Less Costs Of Disposal (Details) $ / shares in Units, CAD in Millions | Dec. 31, 2017 CAD | Dec. 31, 2017 USD ($) $ / shares | Dec. 31, 2016 CAD | Dec. 31, 2016 USD ($) $ / shares | Dec. 31, 2015 USD ($) $ / shares
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Disclosure of fair value measurement of assets [line items] |  |  |  |  |
Coal | CAD | CAD 58 |  | CAD 65 |  |
Minimum |  |  |  |  |
Disclosure of fair value measurement of assets [line items] |  |  |  |  |
Discount rate used in current measurement of fair value less costs of disposal | 5.50% | 5.50% | 5.50% | 5.50% |
Maximum |  |  |  |  |
Disclosure of fair value measurement of assets [line items] |  |  |  |  |
Discount rate used in current measurement of fair value less costs of disposal | 6.00% | 6.00% | 6.00% | 6.00% |
U.S. Coal | Minimum |  |  |  |  |
Disclosure of fair value measurement of assets [line items] |  |  |  |  |
Average power prices | $ / shares |  | $ 21.50 |  | $ 22.00 | $ 24.00
Coal | $ |  | $ 2.08 |  | $ 1.69 | $ 2.44
Discount rate used in current measurement of fair value less costs of disposal | 7.90% | 7.90% | 5.40% | 5.40% | 5.20%
U.S. Coal | Maximum |  |  |  |  |
Disclosure of fair value measurement of assets [line items] |  |  |  |  |
Average power prices | $ / shares |  | $ 34.81 |  | $ 46.00 | $ 50.00
Coal | $ |  | $ 2.29 |  | $ 2.09 | $ 2.90
Discount rate used in current measurement of fair value less costs of disposal | 9.00% | 9.00% | 5.70% | 5.70% | 6.20%

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