v3.26.1
RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Other Financial Instruments (Details) - CAD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Dec. 31, 2025
Fair Value of Other Financial Instruments      
Transfer into or out of level 3 $ 0   $ 0
FMVA investments 188,000,000   185,000,000
Fair value of investments 1,329,000,000   $ 1,293,000,000
Unrealized gain (loss) on investment $ (20,000,000) $ 1,000,000  
Investment, Type [Extensible Enumeration] Debt Securities   Debt Securities
Long-term debt $ 107,400,000,000   $ 102,700,000,000
Level 2      
Fair Value of Other Financial Instruments      
Fair value of investments 426,000,000   416,000,000
Restricted long-term investments, cost basis 1,300,000,000   1,200,000,000
Level 2 | Debt Securities      
Fair Value of Other Financial Instruments      
Fair value of investments 1,300,000,000   1,200,000,000
Carrying Value      
Fair Value of Other Financial Instruments      
Long-term debt 108,500,000,000   $ 104,400,000,000
Net investment hedges      
Fair Value of Other Financial Instruments      
Unrealized foreign exchange (loss) gain on translation of United States dollar denominated debt (248,000,000) 47,000,000  
Realized loss associated with the settlement of foreign exchange forward contracts $ 0 $ (81,000,000)