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RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Schedule of Changes in Net Fair Value of Derivative Assets and Liabilities Classified as Level 3 in the Fair Value Hierarchy (Details) - CAD ($)
$ in Millions
6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Changes in net fair value of derivative assets and liabilities classified as Level 3    
Level 3 net derivative asset/(liability) at beginning of period $ 188 $ (52)
Total gain/(loss), unrealized    
Included in earnings $ 109 42
Fair Value, Net Derivative Asset (Liability), Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Net Income (Loss), Including Portion Attributable to Noncontrolling Interest  
Included in OCI $ 0 (1)
Fair Value, Net Derivative Asset (Liability), Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration] Other Comprehensive Income (Loss), Net of Tax  
Included in regulatory assets/liabilities $ (168) (132)
Settlements 50 160
Level 3 net derivative asset at end of period $ 179 $ 17