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Asset Impairment Charges and Reversals - Significant Inputs (Details)
 12 Months Ended
Dec. 31, 2019
uSDPricePerGallon
uSDPerMega-wattHour
Dec. 31, 2016
uSDPricePerGallon
uSDPerMega-wattHour
Maximum | Mid-Columbia annual average power prices    
Disclosure of impairment loss recognised or reversed [line items]    
Significant unobservable input, price per megawatt hour | uSDPerMega-wattHour 42 46
Maximum | On-highway diesel fuel on coal shipments    
Disclosure of impairment loss recognised or reversed [line items]    
Significant unobservable input, fuel price per gallon | uSDPricePerGallon 2.40 2.09
Maximum | Discount rates    
Disclosure of impairment loss recognised or reversed [line items]    
Significant unobservable input, assets 0.064 0.057
Minimum | Mid-Columbia annual average power prices    
Disclosure of impairment loss recognised or reversed [line items]    
Significant unobservable input, price per megawatt hour | uSDPerMega-wattHour 30 22
Minimum | On-highway diesel fuel on coal shipments    
Disclosure of impairment loss recognised or reversed [line items]    
Significant unobservable input, fuel price per gallon | uSDPricePerGallon 2.35 1.69
Minimum | Discount rates    
Disclosure of impairment loss recognised or reversed [line items]    
Significant unobservable input, assets 0.052 0.054