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Nature of Regulation - FortisAlberta (Details) - FortisAlberta - AUC |  | 12 Months Ended | 36 Months Ended
--- | --- | --- | ---
Jan. 01, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Public Utilities, General Disclosures [Line Items] |  |  |  |
Efficiency gains benefit period beyond initial term (period) |  |  | 2 years |
ROE (percent) |  |  | 8.30% | 8.30%
Capital structure of common equity (percent) | 37.00% |  |  | 40.00%
Forecast |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |
ROE (percent) |  | 8.50% |  |

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