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Nature of Regulation - Regulated Electric Utilities - Caribbean (Details)
CAD in Millions, $ in Millions
 12 Months Ended
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%