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Asset Impairment Charges and Reversals (Tables) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of impairment of assets [Abstract] |  |  |
Disclosure of the estimated the fair value less costs of disposal |
Mid-Columbia annual average power prices | US$21.50 to US$34.81 per MWh
On-highway diesel fuel on coal shipments | US$2.08 to US$2.29 per gallon
Discount rates | 7.9 to 9.0 per cent
| The Corporation estimated the fair value less costs of disposal of the CGU, a Level III fair value measurement, utilizing the Corporation’s long-range forecast and the following key assumptions:
Mid-Columbia annual average power prices | US$22.00 to US$46.00 per MWh
On-highway diesel fuel on coal shipments | US$1.69 to US$2.09 per gallon
Discount rates | 5.4 to 5.7 per cent
| The Corporation estimated the fair value less costs of disposal of the CGU, a Level III fair value measurement, utilizing the Corporation’s long-range forecast and the following key assumptions:
Mid-Columbia annual average power prices | US$24.00 to US$50.00 per MWh
On-highway diesel fuel on coal shipments | US$2.44 to US$2.90 per gallon
Discount rates | 5.2 to 6.2 per cent

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