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Financial Instruments (Details) $ in Millions | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2019 USD ($) MW | Dec. 31, 2019 CAD ($) | Dec. 31, 2018 USD ($) MW | Dec. 31, 2018 CAD ($) | Dec. 31, 2019 CAD ($) MW | Dec. 31, 2018 CAD ($) MW | Dec. 31, 2017 CAD ($)
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |
Risk management assets |  |  |  |  | $ 166 | $ 146 |
Increase (decrease) in derivative forward power prices, reasonable possible | $ 9 |  | $ 6 |  |  |  |
Decrease in base fair value |  |  |  | $ (11) |  |  |
Decrease in sensitivity values |  | $ (2) |  |  |  |  |
Fixed-price contracts | Minimum |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |
Long-term fixed price power sale contract, delivery volume | MW |  |  | 300 |  |  | 300 |
Fixed-price contracts | Maximum |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |
Long-term fixed price power sale contract, delivery volume | MW | 380 |  |  |  | 380 |  |
Unit contingent power purchases | Minimum |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |
Volume discount ranges, percent | 2.20% | 2.20% | 2.20% | 2.20% |  |  |
Sensitivity analysis in derivative, price discount, percent | 1.00% | 1.00% | 1.10% | 1.10% |  |  |
Sensitivity analysis in derivative, volume discount ranges, percent | 8.60% | 8.60% | 8.60% | 8.60% |  |  |
Unit contingent power purchases | Maximum |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |
Volume discount ranges, percent | 2.80% | 2.80% | 16.90% | 16.90% |  |  |
Sensitivity analysis in derivative, price discount, percent | 2.00% | 2.00% | 1.90% | 1.90% |  |  |
Sensitivity analysis in derivative, volume discount ranges, percent | 10.50% | 10.50% | 27.30% | 27.30% |  |  |
Structured products – Eastern US |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |
Sensitivity Analysis In Derivative, Implied Correlations, Percent | 0.00% | 0.00% |  |  |  |  |
Sensitivity Analysis In Derivative, Implied Volatility And Correlation, Percent |  |  | 3000.00% | 3000.00% |  |  |
Structured products – Eastern US | Minimum |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |
Basis relationship, percent | 91.00% | 91.00% | 75.00% | 75.00% |  |  |
Non-standard shape factors, percent | 63.00% | 63.00% | 63.00% | 63.00% |  |  |
Sensitivity analysis in derivative, basis relationship, percent | 4.00% | 4.00% | 4.20% | 4.20% |  |  |
Sensitivity analysis in derivative, non-standard shape factors, percent | 4.00% | 4.00% | 4.00% | 4.00% |  |  |
Implied volatility for shares, significant unobservable inputs, entity's own derivative | 0.00% | 0.00% | 25.00% | 25.00% |  |  |
Implied correlations for shares, significant unobservable inputs, entity's own derivative |  |  | 70.00% | 70.00% |  |  |
Sensitivity Analysis In Derivative, Implied Volatility, Percent | 0.00% | 0.00% | 37.00% | 37.00% |  |  |
Structured products – Eastern US | Maximum |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |
Basis relationship, percent | 112.00% | 112.00% | 109.00% | 109.00% |  |  |
Non-standard shape factors, percent | 116.00% | 116.00% | 104.00% | 104.00% |  |  |
Sensitivity analysis in derivative, basis relationship, percent | 6.00% | 6.00% | 6.90% | 6.90% |  |  |
Sensitivity analysis in derivative, non-standard shape factors, percent | 10.00% | 10.00% | 9.30% | 9.30% |  |  |
Implied volatility for shares, significant unobservable inputs, entity's own derivative |  |  | 84.00% | 84.00% |  |  |
Implied correlations for shares, significant unobservable inputs, entity's own derivative | 0.00% | 0.00% |  |  |  |  |
Sensitivity Analysis In Derivative, Implied Volatility, Percent |  |  | 49.00% | 49.00% |  |  |
Long-term wind energy sale – Eastern US |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |
Sensitivity Analysis In Derivative, Implied Correlations, Percent | 10.00% | 10.00% | 10.00% | 10.00% |  |  |
Long-term contracts | UNITED STATES | Fixed-price contracts | Minimum |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |
Forward power price ranges per MWh | $ 20 |  | $ 20 |  |  |  |
Long-term contracts | UNITED STATES | Fixed-price contracts | Maximum |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |
Forward power price ranges per MWh | 28 |  | 35 |  |  |  |
Level III | Long-term contracts | Fixed-price contracts |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |
Forward power price ranges per MWh | 1 |  | 1 |  |  |  |
Level III | Long-term contracts | Fixed-price contracts | Minimum |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |
Forward power price ranges per MWh |  |  | 7 |  |  |  |
Level III | Long-term contracts | Fixed-price contracts | Maximum |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |
Forward power price ranges per MWh | 9 |  | 8 |  |  |  |
Level III | Long-term contracts | Forward contract |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |
Forward power price ranges per MWh | 6 |  | 6 |  |  |  |
Level III | Long-term contracts | Forward contract | Minimum |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |
Forward power price ranges per MWh | 38 |  | 42 |  |  |  |
Level III | Long-term contracts | Forward contract | Maximum |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |
Forward power price ranges per MWh | $ 60 |  | $ 68 |  |  |  |
Recurring fair value measurement | Level I |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |
Risk exposure associated with instruments sharing characteristic |  |  |  |  | $ (3) | $ 3 |
Recurring fair value measurement | Level III |  |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |  |
Risk exposure associated with instruments sharing characteristic |  |  |  |  | $ 686 | $ 695 | $ 771

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