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Significant Events - Transition to Clean Power in Alberta (Details) MMcf in Millions, CAD in Millions |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 06, 2017 km MMcf | Jun. 30, 2017 CAD | Dec. 31, 2017 pipeline MW
Investment [Line Items] |  |  |
Number of pipelines needed to reduce operational risk (at least) | pipeline |  |  | 2
Sundance Power Plant |  |  |
Investment [Line Items] |  |  |
Capacity of facility (in megawatts) | MW |  |  | 2,141
Sundance Units 1 and 2 |  |  |
Investment [Line Items] |  |  |
Capacity of facility (in megawatts) | MW |  |  | 560
Sundance Unit 1 |  |  |
Investment [Line Items] |  |  |
Impairment loss | CAD |  | CAD 20 |
Sundance Unit 2 |  |  |
Investment [Line Items] |  |  |
Transition to Gas and Renewables Generation, Mothballing Term | 2 years |  |
Sundance Unit 3 |  |  |
Investment [Line Items] |  |  |
Transition to Gas and Renewables Generation, Mothballing Term | 2 years |  |
Sundance Unit 4 |  |  |
Investment [Line Items] |  |  |
Transition to Gas and Renewables Generation, Mothballing Term | 2 years |  |
Sundance Unit 5 |  |  |
Investment [Line Items] |  |  |
Transition to Gas and Renewables Generation, Mothballing Term | 1 year |  |
Coal-fired fleet |  |  |
Investment [Line Items] |  |  |
PP&E useful life |  |  | 75 years
Tidewater | Tidewater Brazeau River Pipeline |  |  |
Investment [Line Items] |  |  |
Natural gas pipeline, length (km) | km | 120 |  |
Natural gas pipeline capacity (mmcf) | MMcf | 130 |  |
Natural gas pipeline capacity, expansion capability (mmcf) | MMcf | 340 |  |
Future option to acquire pipeline, ownership percentage | 50.00% |  |
Minimum | Gas conversion units |  |  |
Investment [Line Items] |  |  |
PP&E useful life |  |  | 5 years
Maximum | Gas conversion units |  |  |
Investment [Line Items] |  |  |
PP&E useful life |  |  | 10 years

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