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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
9 Months Ended
Sep. 30, 2026
Sep. 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 $ 189 $ (52)
Total gain/(loss), unrealized    
Included in earnings $ 94 84
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 $ (138) 44
Settlements 40 115
Level 3 net derivative asset at end of period $ 185 $ 190