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Financial Instruments - Sensitivity Ranges for the Base Fair Values (Details) | 12 Months Ended
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Dec. 31, 2022 CAD ($) | Dec. 31, 2022 USD ($) | Dec. 31, 2021 CAD ($) | Dec. 31, 2021 USD ($)
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 |  |  |  | $ 1
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) |  | 6
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 |  | (2) |  |
Minimum | Long-term power sale – US | Long-term contracts |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity | $ 163,000,000 |  | $ 145,000,000 |
Increase (decrease) in derivative future power prices, reasonable possible |  | (5) |  | $ (3)
Minimum | Coal transportation – US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity | $ 13,000,000 |  | $ 18,000,000 |
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 |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in derivative future power prices, reasonable possible |  | $ (5) |  |
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 | $ 21,000,000 |  | $ (9,000,000) |
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 |  | $ (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 | 0.95 | 0.95
Minimum | Long-term wind energy sale – Eastern US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity | $ 18,000,000 |  | $ 16,000,000 |
Increase (decrease) in monthly wind discounts expressed as a percentage | 0.00% | 0.00% |  |
Minimum | Long-term wind energy sale – Canada |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity | $ 25,000,000 |  | $ 11,000,000 |
Increase (decrease) in monthly wind discounts expressed as a percentage | 28.00% | 28.00% | (5.00%) | (5.00%)
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 | $ 85 |  | $ (24) |
Minimum | Long-term wind energy sale - Central US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity | $ 28,000,000 |  | $ 15,000,000 |
Increase (decrease) in monthly wind discounts expressed as a percentage | 2.00% | 2.00% | (3.00%) | (3.00%)
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 |  |  |  | $ (2)
Minimum | Others |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity | $ 19,000,000 |  | $ 6,000,000 |
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 |  |  |  | (3)
Maximum | Long-term power sale – US | Long-term contracts |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity | 15,000,000 |  | 22,000,000 |
Increase (decrease) in derivative future power prices, reasonable possible |  | $ 55 |  | $ 20
Maximum | Coal transportation – US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity | $ 14,000,000 |  | $ 3,000,000 |
Maximum | Coal transportation – US | US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in rail rate escalation expressed as a percentage |  |  | 0.04 | 0.04
Maximum | Coal transportation – US | Long-term contracts |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in derivative future power prices, reasonable possible |  | $ 55 |  |
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 |  | $ 9,000,000 |
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 |  | $ 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 | 1.05 | 1.05
Maximum | Long-term wind energy sale – Eastern US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity | $ 22,000,000 |  | $ 17,000,000 |
Increase (decrease) in monthly wind discounts expressed as a percentage | 5.00% | 5.00% |  |
Maximum | Long-term wind energy sale – Canada |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity | $ 47,000,000 |  | $ 21,000,000 |
Increase (decrease) in monthly wind discounts expressed as a percentage | 5.00% | 5.00% | 5.00% | 5.00%
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] |  |  |  |
Sensitivity | $ 74,000,000 |  | $ 27,000,000 |
Increase (decrease) in monthly wind discounts expressed as a percentage | 5.00% | 5.00% | 3.00% | 3.00%
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) |  | $ (3)
Maximum | Others |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity | $ 18,000,000 |  | $ 6,000,000 |
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 |  |  |  | $ 2

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