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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% |  |

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