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Financial Instruments - Sensitivity ranges for the base fair values (Details) | 12 Months Ended
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Dec. 31, 2021 CAD ($) | Dec. 31, 2021 USD ($) | Dec. 31, 2020 CAD ($) | Dec. 31, 2020 USD ($)
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 |  | $ 1
Fixed-price contracts | Long-term contracts | Level III |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Future power price ranges per MWh |  |  |  | 1
Forward contract | Long-term contracts | Level III |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Future power price ranges per MWh |  |  |  | 6
Increase (decrease) in derivative future power prices, reasonable possible |  | (6) |  |
Minimum | Long-term power sale – US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | $ (5,000,000) |  |  |
Minimum | Long-term power sale – US | Long-term contracts |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | 145,000,000 |  | $ 59,000,000 |
Increase (decrease) in derivative future power prices, reasonable possible |  | (3) |  | $ 3
Minimum | Coal transportation – US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | $ 18,000,000 |  |  |
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 |  | $ (3) |  |
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 | Historical volatility for shares, measurement input [member] |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in significant unobservable inputs, derivative asset | 0.80 | 0.80 |  |
Minimum | Full requirements – Eastern US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | $ 9,000,000 |  |  |
Minimum | Full requirements – Eastern US | US | Volume Rate |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in significant unobservable inputs, derivative asset | 0.95 | 0.95 |  | 0.95
Minimum | Long-term wind energy sale – Eastern US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | $ 16,000,000 |  |  |
Minimum | Long-term wind energy sale – Eastern US | Long-term contracts |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments |  |  | 22,000,000 |
Minimum | Long-term wind energy sale – Canada |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | $ 11,000,000 |  |  |
Increase (decrease) in monthly wind discounts expressed as a percentage | 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 | $ 0.24 |  |  |
Minimum | Long-term wind energy sale - Central US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | $ 15,000,000 |  |  |
Increase (decrease) in monthly wind discounts expressed as a percentage | 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 ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | $ 6,000,000 |  | $ 5,000,000 |
Minimum | Coal transportation – US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity analysis in derivative, basis relationship, percent |  |  | 80.00% | 80.00%
Sensitivity analysis in derivative, non-standard shape factors, percent |  |  | 0.00% | 0.00%
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 |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments |  |  | $ 3,000,000 |
Maximum | Long-term power sale – US | Long-term contracts |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | 22,000,000 |  | 35,000,000 |
Increase (decrease) in derivative future power prices, reasonable possible |  | 20 |  | $ 5
Maximum | Coal transportation – US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | $ 3,000,000 |  |  |
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 |  | $ 20 |  |
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 | 0.04 | 0.04 |  |
Maximum | Coal transportation – US | Long-term contracts | US | Historical volatility for shares, measurement input [member] |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in significant unobservable inputs, derivative asset | 1.20 | 1.20 |  |
Maximum | Full requirements – Eastern US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | $ 9,000,000 |  | 3,000,000 |
Maximum | Full requirements – Eastern US | US | Volume Rate |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Increase (decrease) in significant unobservable inputs, derivative asset | 1.05 | 1.05 |  | 1.05
Maximum | Long-term wind energy sale – Eastern US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | $ 17,000,000 |  |  |
Maximum | Long-term wind energy sale – Eastern US | Long-term contracts |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments |  |  | 22,000,000 |
Maximum | Long-term wind energy sale – Canada |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | $ 21,000,000 |  |  |
Increase (decrease) in monthly wind discounts expressed as a percentage | 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 | $ (0.05) |  |  |
Maximum | Long-term wind energy sale - Central US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | $ 27,000,000 |  |  |
Increase (decrease) in monthly wind discounts expressed as a percentage | 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 |  | $ (3) |  |
Maximum | Others |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption increase (decrease), financial instruments | $ 6,000,000 |  | $ 5,000,000 |
Maximum | Coal transportation – US |  |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |  |
Sensitivity analysis in derivative, basis relationship, percent |  |  | 120.00% | 120.00%
Sensitivity analysis in derivative, non-standard shape factors, percent |  |  | 4.00% | 4.00%
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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