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Financial Instruments - Sensitivity Ranges for the Base Fair Values (Details) | 12 Months Ended
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Dec. 31, 2023 CAD ($) | Dec. 31, 2023 USD ($) | Dec. 31, 2022 CAD ($) | Dec. 31, 2022 USD ($)
Long-term wind energy sale – Eastern US | Long-term contracts | Level III |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in derivative future power prices, reasonable possible |  |  |  | $ 6
Long-term wind energy sale - Central US | Long-term contracts | Level III |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in derivative future power prices, reasonable possible |  |  | $ 2 |
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 | Long-term power sale – US | Long-term contracts |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in derivative future power prices, reasonable possible |  |  |  | $ (5)
Sensitivity |  |  | 163,000,000 |
Minimum | Coal transportation – US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity | $ 4,000,000 |  | $ 13,000,000 |
Minimum | Coal transportation – US | US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in rail rate escalation expressed as a percentage |  |  | 0 | 0
Minimum | Coal transportation – US | Long-term contracts | US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in rail rate escalation expressed as a percentage | 0 | 0 |  |
Minimum | Coal transportation – US | Long-term contracts | US | 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 | Full requirements – Eastern US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity | $ 3,000,000 |  | $ (21,000,000) |
Minimum | Full requirements – Eastern US | Cost of supply |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in derivative future power prices, reasonable possible |  | $ (2.30) |  | $ 0.50
Minimum | Full requirements – Eastern US | US | Volume |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in significant unobservable inputs, derivative asset | 0.96 | 0.96 | 0.96 | 0.96
Minimum | Long-term wind energy sale – Eastern US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in monthly wind discounts expressed as a percentage | 0.00% | 0.00% |  |
Sensitivity | $ 28,000,000 |  | $ 18,000,000 |
Minimum | Long-term wind energy sale – Eastern US | 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 | 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 | 16.00% | 16.00% | (5.00%) | (5.00%)
Sensitivity | $ 23,000,000 |  | $ 25,000,000 |
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 | $ 81 |  | $ 85 |
Minimum | Long-term wind energy sale - Central US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in monthly wind discounts expressed as a percentage | 5.00% | 5.00% | 2.00% | 2.00%
Sensitivity | $ 36,000,000 |  | $ 28,000,000 |
Minimum | Long-term wind energy sale - Central US | Long-term contracts | Level III |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in derivative future power prices, reasonable possible |  | $ 1 |  |
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) |  | $ (2)
Maximum | Long-term power sale – US | Long-term contracts |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in derivative future power prices, reasonable possible |  |  |  | $ 55
Sensitivity |  |  | 15,000,000 |
Maximum | Coal transportation – US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity | $ 6,000,000 |  | $ 14,000,000 |
Maximum | Coal transportation – US | US |  |  |  |
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 – US | Long-term contracts | US |  |  |  |
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 – US | Long-term contracts | US | 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 | Full requirements – Eastern US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity | $ 3,000,000 |  | $ 3,000,000 |
Maximum | Full requirements – Eastern US | Cost of supply |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in derivative future power prices, reasonable possible |  | $ 2.40 |  | $ 3.30
Maximum | Full requirements – Eastern US | US | Volume |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in significant unobservable inputs, derivative asset | 1.04 | 1.04 | 1.04 | 1.04
Maximum | Long-term wind energy sale – Eastern US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in monthly wind discounts expressed as a percentage | 9.00% | 9.00% |  |
Sensitivity | $ 24,000,000 |  | $ 22,000,000 |
Maximum | Long-term wind energy sale – Eastern US | Long-term contracts | Level III |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in derivative future power prices, reasonable possible |  |  |  | $ 0.05
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% | (28.00%) | (28.00%)
Sensitivity | $ 65,000,000 |  | $ 47,000,000 |
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 | $ (5) |  | $ (5) |
Maximum | Long-term wind energy sale - Central US |  |  |  |
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 | $ 81,000,000 |  | $ 74,000,000 |
Maximum | Long-term wind energy sale - Central US | Long-term contracts | Level III |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in derivative future power prices, reasonable possible |  | $ (2) |  |
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 |  |

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