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Description of Business - Electric & Gas Utilities - United States (Details)
km in Thousands
 12 Months Ended
Dec. 31, 2016
station
county
km
MW
Oct. 31, 2016
Oct. 14, 2016
station
ITC      
Public Utilities, General Disclosures [Line Items]      
Peak transmission line capacity (MW) (more than) 26,000    
Area of operation (km) | km 25    
Number of third-party generating stations with transmission assets | station 570   570
TEP and UNS Electric, Inc      
Public Utilities, General Disclosures [Line Items]      
Generating capacity (MW) 2,994    
TEP and UNS Electric, Inc | Solar      
Public Utilities, General Disclosures [Line Items]      
Generating capacity (MW) 54    
TEP and UNS Electric, Inc | Coal      
Public Utilities, General Disclosures [Line Items]      
Generating Capacity, Percent 47.00%    
Central Hudson      
Public Utilities, General Disclosures [Line Items]      
Number of countries in which entity operates | county 8    
Central Hudson | Gas-Fired and Hydroelectric Power Generation      
Public Utilities, General Disclosures [Line Items]      
Generating capacity (MW) 64    
ITC      
Public Utilities, General Disclosures [Line Items]      
Controlling ownership interest (percent)   80.10% 80.10%
Noncontrolling ownership (percent)   19.90% 19.90%