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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
3 Months Ended
Mar. 31, 2026
Mar. 31, 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 $ (101) 23
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 2
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 $ (107) (45)
Settlements 33 129
Level 3 net derivative asset at end of period $ 13 $ 57