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Financial Instruments - Sensitivity Ranges for the Base Fair Values (Details) | 12 Months Ended
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Dec. 31, 2024 CAD ($) | Dec. 31, 2024 USD ($) | Dec. 31, 2023 CAD ($) | Dec. 31, 2023 USD ($)
Forward contract | Long-term contracts | Level III |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in derivative future power prices, reasonable possible |  | $ (6) |  |
Minimum | Coal transportation – U.S. |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity | $ 1,000,000 |  | $ 4,000,000 |
Minimum | Coal transportation – U.S. | U.S. |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in rail rate escalation expressed as a percentage |  |  | 0 | 0
Minimum | Coal transportation – U.S. | Long-term contracts | U.S. |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in rail rate escalation expressed as a percentage | 0 | 0 |  |
Minimum | Coal transportation – U.S. | Long-term contracts | U.S. | Volatility |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in significant unobservable inputs, derivative asset | 0.80 | 0.80 | 0.80 | 0.80
Minimum | Long-term wind energy sale — Eastern U.S. |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in monthly wind discounts expressed as a percentage | 0.00% | 0.00% |  |
Sensitivity | $ 30,000,000 |  | $ 28,000,000 |
Minimum | Long-term wind energy sale — Eastern U.S. | Long-term contracts | Level III |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in derivative future power prices, reasonable possible |  |  |  | $ 0
Minimum | Fixed-price contracts | Long-term contracts | Level III |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in derivative future power prices, reasonable possible |  | $ (12) |  | $ (12)
Minimum | Long-term wind energy sale — Canada |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in monthly wind discounts expressed as a percentage | 15.00% | 15.00% | (16.00%) | (16.00%)
Sensitivity | $ 17,000,000 |  | $ 23,000,000 |
Minimum | Long-term wind energy sale — Canada | Long-term contracts | Level III |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in derivative future power prices, reasonable possible | $ 57 |  | $ (81) |
Minimum | Long-term wind energy sale — Central U.S. |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in monthly wind discounts expressed as a percentage | 2.00% | 2.00% | (5.00%) | (5.00%)
Sensitivity | $ 77,000,000 |  | $ 36,000,000 |
Minimum | Long-term wind energy sale — Central U.S. | Long-term contracts | Level III |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in derivative future power prices, reasonable possible |  | $ 4 | (1) |
Maximum | Coal transportation – U.S. |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity | $ 1,000,000 |  | $ 6,000,000 |
Maximum | Coal transportation – U.S. | U.S. |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in rail rate escalation expressed as a percentage |  |  | 0.10 | 0.10
Maximum | Coal transportation – U.S. | Long-term contracts | U.S. |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in rail rate escalation expressed as a percentage | 0.10 | 0.10 |  |
Maximum | Coal transportation – U.S. | Long-term contracts | U.S. | Volatility |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in significant unobservable inputs, derivative asset | 1.20 | 1.20 | 1.20 | 1.20
Maximum | Long-term wind energy sale — Eastern U.S. |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in monthly wind discounts expressed as a percentage | 6.00% | 6.00% |  |
Sensitivity | $ 42,000,000 |  | $ 24,000,000 |
Maximum | Long-term wind energy sale — Eastern U.S. | Level III |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in derivative future power prices, reasonable possible |  |  |  | $ 6
Maximum | Long-term wind energy sale — Eastern U.S. | Long-term contracts | Level III |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in derivative future power prices, reasonable possible |  |  |  | 0.09
Maximum | Fixed-price contracts | Long-term contracts | Level III |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in derivative future power prices, reasonable possible |  | $ 8 |  | $ 8
Maximum | Long-term wind energy sale — Canada |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in monthly wind discounts expressed as a percentage | 5.00% | 5.00% | 5.00% | 5.00%
Sensitivity | $ 53,000,000 |  | $ 65,000,000 |
Maximum | Long-term wind energy sale — Canada | Long-term contracts | Level III |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in derivative future power prices, reasonable possible | $ (10) |  | $ 5 |
Maximum | Long-term wind energy sale — Central U.S. |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in monthly wind discounts expressed as a percentage | 2.00% | 2.00% | 2.00% | 2.00%
Sensitivity | $ 84,000,000 |  | $ 81,000,000 |
Maximum | Long-term wind energy sale — Central U.S. | Long-term contracts | Level III |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in derivative future power prices, reasonable possible |  | $ 3 | $ 2 |

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