v3.26.1
Fair Value of Financial Instruments and Risk Management - Derivative Narrative (Details)
$ in Millions, $ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
CAD ($)
Jun. 30, 2026
USD ($)
Mar. 31, 2026
USD ($)
Jun. 30, 2025
CAD ($)
Jun. 30, 2025
USD ($)
Jun. 30, 2026
CAD ($)
Jun. 30, 2026
USD ($)
Jun. 30, 2025
CAD ($)
Jun. 30, 2025
USD ($)
Jun. 30, 2026
USD ($)
Dec. 31, 2025
CAD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                      
Total regulatory assets $ 5,206         $ 5,206         $ 5,022
Total regulatory liabilities 4,458         $ 4,458         4,262
UNS Energy                      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                      
Realized gains, portion shared with customers (percent)           10.00% 10.00%        
Fortis | Unsecured senior notes, 2029 maturity | Unsecured                      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                      
Debt instrument, face amount $ 500         $ 500          
Interest rate 4.43%         4.43%       4.43%  
Energy contracts subject to regulatory deferral | Derivatives                      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                      
Total regulatory liabilities $ 15         $ 15         37
Energy contracts subject to regulatory deferral | Derivatives                      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                      
Total regulatory assets 129         129         $ 135
Energy contracts not subject to regulatory deferral                      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                      
Unrealized gains (losses) on energy contracts 2     $ 1   24   $ 32      
Total return swaps | Fortis                      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                      
Notional amount 92         92          
Gains (losses) on derivatives 7     (1)   $ 18   9      
Total return swaps | Fortis | Maximum                      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                      
Derivative terms           3 years 3 years        
Foreign exchange contracts | Fortis                      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                      
Notional amount                   $ 531  
Gains (losses) on derivatives (7)     18   $ (12)   19      
Interest rate swap | ITC                      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                      
Notional amount     $ 705             150  
Derivative terms           5 years 5 years        
Derivatives, period over which gains (losses) will be reclassified to earnings           5 years 5 years        
Gains (losses) on derivatives   $ 1     $ (4)   $ 2   $ (8)    
Realized gain on derivatives     $ 4                
Foreign exchange contracts and cross-currency interest rate swaps | Fortis                      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                      
Notional amount                   $ 391  
Gains (losses) on derivatives $ (5)     $ 21   $ (13)   $ 17      
Derivative interest rate 4.34%         4.34%       4.34%