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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
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%