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Fair Value of Financial Instruments and Risk Management - Derivatives (Narrative) (Details)
$ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2025
USD ($)
Dec. 31, 2024
CAD ($)
Dec. 31, 2024
USD ($)
Dec. 31, 2023
CAD ($)
Dec. 31, 2024
USD ($)
Sep. 30, 2024
CAD ($)
May 31, 2024
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Unrealized losses recognized in regulatory assets   $ 4,631   $ 4,384      
Unrealized gains recognized as regulatory liabilities   $ 4,291   3,958      
UNS Energy              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Realized gains shared with customers, percent   10.00% 10.00%        
ITC | Unsecured Senior Notes, 2034 Maturity | Unsecured              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Face value         $ 400   $ 400
Stated interest rate             5.65%
Fortis | Unsecured Senior Notes, May Issuance, 2029 Maturity | Unsecured              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Face value   $ 500          
Stated interest rate   4.43%     4.43%    
Energy contracts subject to regulatory deferral | Derivative instruments              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Unrealized gains recognized as regulatory liabilities   $ 41   37      
Energy contracts subject to regulatory deferral | Derivative instruments              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Unrealized losses recognized in regulatory assets   175   197      
Energy contracts not subject to regulatory deferral              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Unrealized gain (loss) recognized in revenue   48   (28)      
Total return swaps              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Notional amount of derivative   $ 134          
Derivative term   3 years 3 years        
Unrealized gain (loss) recognized in other income, net   $ 12   0      
Foreign exchange contracts              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Notional amount of derivative   608          
Unrealized gain (loss) recognized in other income, net   $ (17)   10      
Interest rate locks | ITC              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Notional amount of derivative             $ 300
Realized gain (loss) recognized in other comprehensive income     $ (3)        
Gain on derivative, reclassification adjustment period   5 years 5 years        
Interest rate locks | Fortis              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Notional amount of derivative           $ 250  
Realized gain (loss) recognized in other comprehensive income   $ (2)          
Gain on derivative, reclassification adjustment period   7 years 7 years        
Interest rate locks | Fortis | Unsecured Senior Notes, November Issuance, 2023 Maturity              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Face value           $ 500  
Interest rate swaps | ITC              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Notional amount of derivative         $ 135    
Derivative term   5 years 5 years        
Realized gain (loss) recognized in other comprehensive income     $ 4        
Interest rate swaps | ITC | Forecast              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Notional amount of derivative $ 230            
2025 interest rate swaps | ITC | Forecast              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Notional amount of derivative $ 95            
Derivative term 5 years            
Cross currency interest rate swaps | Fortis              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Notional amount of derivative         $ 391    
Unrealized gain (loss) recognized in other income, net   $ (29)   $ 15      
Interest rate   4.34%     4.34%