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Nature of Regulation - Regulated Electric Utilities - Caribbean (Details) CAD in Millions, $ in Millions | 12 Months Ended
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Dec. 31, 2016 USD ($) license | Dec. 31, 2016 CAD license | Dec. 31, 2015
Caribbean Utilities | Government of the Cayman Islands |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
T & D license term | 20 years | 20 years |
Generation license term | 25 years | 25 years |
Caribbean Utilities | Minimum | Government of the Cayman Islands |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
ROA (percent) | 6.75% | 6.75% | 7.25%
Caribbean Utilities | Maximum | Government of the Cayman Islands |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
ROA (percent) | 8.75% | 8.75% | 9.25%
Fortis Turks and Caicos | Government of the Turks and Caicos Islands |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Licenses (number) | 2 | 2 |
Operating license term | 50 years | 50 years |
Allowable Operating Profit | $ 44 | CAD 58 |
Cumulative Shortfall | $ 236 | CAD 317 |
Fortis Turks and Caicos | Minimum | Government of the Turks and Caicos Islands |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
ROA (percent) | 15.00% | 15.00% |
Fortis Turks and Caicos | Maximum | Government of the Turks and Caicos Islands |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
ROA (percent) | 17.50% | 17.50% |

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