v3.25.4
DEBT AND CREDIT AGREEMENTS - Summary of Outstanding Long-Term Debt (Details) - CAD ($)
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Oct. 31, 2023
Debt Instrument [Line Items]        
Long-term debt, face amount $ 19,045,000,000      
Long-term debt, carrying amount 19,045,000,000 $ 17,495,000,000    
Add: Net unamortized debt premiums 39,000,000 41,000,000    
Add: Realized mark-to-market gain 0 3,000,000    
Less: Unamortized deferred debt issuance costs (67,000,000) (60,000,000)    
Total long-term debt 19,017,000,000 17,479,000,000    
Proceeds from long-term debt offering 0 0    
Interest Rate Swap        
Debt Instrument [Line Items]        
Add: Realized mark-to-market gain     $ 6,000,000  
Fixed-to-floating interest-rate swap       $ 400,000,000
Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, face amount 18,620,000,000 17,070,000,000    
Proceeds from long-term debt offering 2,700,000,000 2,750,000,000    
Long-term debt repaid $ 1,150,000,000 $ 700,000,000    
1.76% Series 45 notes due 2025 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 1.76% 1.76%    
Long-term debt, face amount $ 0 $ 400,000,000    
2.97% Series 40 notes due 2025 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 2.97% 2.97%    
Long-term debt, face amount $ 0 $ 350,000,000    
5.54% Series 57 notes due 20251 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 5.54% 5.54%    
Long-term debt, face amount $ 0 $ 400,000,000    
2.77% Series 35 notes due 2026 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 2.77% 2.77%    
Long-term debt, face amount $ 500,000,000 $ 500,000,000    
Floating-Rate Series 56 Notes Due 2026 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, face amount $ 425,000,000 $ 425,000,000    
4.91% Series 52 notes due 2028 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 4.91% 4.91%    
Long-term debt, face amount $ 750,000,000 $ 750,000,000    
3.02% Series 43 notes due 2029 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 3.02% 3.02%    
Long-term debt, face amount $ 550,000,000 $ 550,000,000    
3.93% Series 53 notes due 2029 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 3.93% 3.93%    
Long-term debt, face amount $ 550,000,000 $ 550,000,000    
2.16% Series 46 notes due 2030 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 2.16% 2.16%    
Long-term debt, face amount $ 400,000,000 $ 400,000,000    
7.35% Debentures due 2030 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 7.35% 7.35%    
Long-term debt, face amount $ 400,000,000 $ 400,000,000    
1.69% Series 49 notes due 2031 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 1.69% 1.69%    
Long-term debt, face amount $ 400,000,000 $ 400,000,000    
2.23% Series 50 notes due 2031 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 2.23% 2.23%    
Long-term debt, face amount $ 450,000,000 $ 450,000,000    
6.93% Series 2 notes due 2032 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 6.93% 6.93%    
Long-term debt, face amount $ 500,000,000 $ 500,000,000    
3.94% Series 61 notes due 2032 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 3.94% 3.94%    
Long-term debt, face amount $ 450,000,000 $ 0    
4.16% Series 54 notes due 2033 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 4.16% 4.16%    
Long-term debt, face amount $ 450,000,000 $ 450,000,000    
3.90% Series 64 notes due 2033 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 3.90% 3.90%    
Long-term debt, face amount $ 1,200,000,000 $ 0    
6.35% Series 4 notes due 2034 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 6.35% 6.35%    
Long-term debt, face amount $ 385,000,000 $ 385,000,000    
4.39% Series 59 notes due 2034 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 4.39% 4.39%    
Long-term debt, face amount $ 550,000,000 $ 550,000,000    
4.25% Series 60 notes due 2035 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 4.25% 4.25%    
Long-term debt, face amount $ 1,075,000,000 $ 1,075,000,000    
4.30% Series 62 notes due 2035 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 4.30% 4.30%    
Long-term debt, face amount $ 300,000,000 $ 0    
5.36% Series 9 notes due 2036 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 5.36% 5.36%    
Long-term debt, face amount $ 600,000,000 $ 600,000,000    
4.89% Series 12 notes due 2037 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 4.89% 4.89%    
Long-term debt, face amount $ 400,000,000 $ 400,000,000    
6.03% Series 17 notes due 2039 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 6.03% 6.03%    
Long-term debt, face amount $ 300,000,000 $ 300,000,000    
5.49% Series 18 notes due 2040 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 5.49% 5.49%    
Long-term debt, face amount $ 500,000,000 $ 500,000,000    
4.39% Series 23 notes due 2041 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 4.39% 4.39%    
Long-term debt, face amount $ 300,000,000 $ 300,000,000    
6.59% Series 5 notes due 2043 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 6.59% 6.59%    
Long-term debt, face amount $ 315,000,000 $ 315,000,000    
4.59% Series 29 notes due 2043 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 4.59% 4.59%    
Long-term debt, face amount $ 435,000,000 $ 435,000,000    
4.17% Series 32 notes due 2044 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 4.17% 4.17%    
Long-term debt, face amount $ 350,000,000 $ 350,000,000    
5.00% Series 11 notes due 2046 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 5.00% 5.00%    
Long-term debt, face amount $ 325,000,000 $ 325,000,000    
3.91% Series 36 notes due 2046 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 3.91% 3.91%    
Long-term debt, face amount $ 350,000,000 $ 350,000,000    
3.72% Series 38 notes due 2047 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 3.72% 3.72%    
Long-term debt, face amount $ 450,000,000 $ 450,000,000    
3.63% Series 41 notes due 2049 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 3.63% 3.63%    
Long-term debt, face amount $ 750,000,000 $ 750,000,000    
2.71% Series 47 notes due 2050 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 2.71% 2.71%    
Long-term debt, face amount $ 500,000,000 $ 500,000,000    
3.64% Series 44 notes due 2050 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 3.64% 3.64%    
Long-term debt, face amount $ 250,000,000 $ 250,000,000    
3.10% Series 51 notes due 2051 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 3.10% 3.10%    
Long-term debt, face amount $ 450,000,000 $ 450,000,000    
4.00% Series 24 notes due 2051 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 4.00% 4.00%    
Long-term debt, face amount $ 225,000,000 $ 225,000,000    
4.46% Series 55 notes due 2053 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 4.46% 4.46%    
Long-term debt, face amount $ 675,000,000 $ 675,000,000    
4.85% Series 58 notes due 2054 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 4.85% 4.85%    
Long-term debt, face amount $ 1,000,000,000 $ 1,000,000,000    
4.95% Series 63 notes due 2055 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 4.95% 4.95%    
Long-term debt, face amount $ 350,000,000 $ 0    
4.80% Series 65 notes due 2056 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 4.80% 4.80%    
Long-term debt, face amount $ 400,000,000 $ 0    
3.79% Series 26 notes due 2062 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 3.79% 3.79%    
Long-term debt, face amount $ 310,000,000 $ 310,000,000    
4.29% Series 30 notes due 2064 | Medium Term Note Program        
Debt Instrument [Line Items]        
Long-term debt, interest rate 4.29% 4.29%    
Long-term debt, face amount $ 50,000,000 $ 50,000,000    
1.41% Series 2020-1 Notes Due 2027        
Debt Instrument [Line Items]        
Long-term debt, interest rate 1.41% 1.41%    
Long-term debt, face amount $ 425,000,000 $ 425,000,000    
Universal Base Shelf Prospectus        
Debt Instrument [Line Items]        
Long-term debt, face amount 425,000,000 425,000,000    
Proceeds from long-term debt offering 0 0    
Long-term debt repaid $ 0 $ 0    
Series 57 Notes Due 2025 | Medium Term Note Program | Interest Rate Swap        
Debt Instrument [Line Items]        
Fixed-to-floating interest-rate swap       $ 400,000,000