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Financial Instruments (Details) | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 USD ($) | Dec. 31, 2016 CAD MW | Dec. 31, 2016 USD ($) | Dec. 31, 2017 CAD MW | Dec. 31, 2017 USD ($) MW | Dec. 31, 2016 USD ($) MW
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Risk management assets (Notes 13 and 14) |  | CAD 249,000,000 |  | CAD 219,000,000 |  |
Increase (Decrease) in Derivative Forward Power Prices, Reasonable Possible | $ | $ 6 |  | $ 5 |  |  |
Decrease in base fair value |  | 50,000,000 |  |  |  |
Decrease in sensitivity values |  | CAD 8,000,000 |  |  |  |
Fixed-price contracts [member] | Maximum |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Long-term Fixed Price Power Sale Contract, Delivery Volume | MW |  |  |  | 380 | 380 |
Fixed-price contracts [member] | Minimum |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Long-term Fixed Price Power Sale Contract, Delivery Volume | MW |  | 300 |  |  |  | 300
Unit Contingent Power Purchases |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Sensitivity analysis in derivative, price discount, percent | 1.10% | 0.75% | 0.75% |  |  |
Sensitivity analysis in derivative, volume discount ranges, percent | 1.94% | 15.50% | 15.50% |  |  |
Unit Contingent Power Purchases | Maximum |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Volume discount ranges, percent | 2.76% | 3.62% | 3.62% |  |  |
Sensitivity analysis in derivative, volume discount ranges, percent | 10.46% |  |  |  |  |
Unit Contingent Power Purchases | Minimum |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Volume discount ranges, percent | 2.20% | 2.15% | 2.15% |  |  |
Sensitivity analysis in derivative, volume discount ranges, percent | 7.77% |  |  |  |  |
Structured Products - Eastern U.S. |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Sensitivity analysis in derivative, basis relationship, percent | 7.00% | 5.00% | 5.00% |  |  |
Sensitivity analysis in derivative, non-standard shape factors, percent | 6.00% | 9.00% | 9.00% |  |  |
Sensitivity Analysis In Derivative, Implied Correlations, Percent | 1000.00% |  |  |  |  |
Sensitivity Analysis In Derivative, Implied Volatility And Correlation, Percent |  | 1000.00% | 1000.00% |  |  |
Structured Products - Eastern U.S. | Maximum |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Basis relationship, percent | 159.00% | 128.00% | 128.00% |  |  |
Non-standard shape factors, percent | 88.00% | 88.00% | 88.00% |  |  |
Implied volatility for shares, significant unobservable inputs, entity's own derivative | 54.00% | 54.00% | 54.00% |  |  |
Sensitivity Analysis In Derivative, Implied Volatility, Percent | 32.00% |  |  |  |  |
Structured Products - Eastern U.S. | Minimum |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Basis relationship, percent | 75.00% | 66.00% | 66.00% |  |  |
Non-standard shape factors, percent | 71.00% | 65.00% | 65.00% |  |  |
Implied volatility for shares, significant unobservable inputs, entity's own derivative | 18.00% | 20.00% | 20.00% |  |  |
Implied correlations for shares, significant unobservable inputs, entity's own derivative | 70.00% | 70.00% | 70.00% |  |  |
Sensitivity Analysis In Derivative, Implied Volatility, Percent | 27.00% |  |  |  |  |
Long-term contracts | UNITED STATES | Fixed-price contracts [member] | Maximum |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Forward power price ranges per MWh | $ |  |  |  |  | $ 34 | $ 36
Long-term contracts | UNITED STATES | Fixed-price contracts [member] | Minimum |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Forward power price ranges per MWh | $ |  |  |  |  | $ 25 | $ 27
Long-term contracts | Canada | Fixed-price contracts [member] |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Long-term Fixed Price Power Sale Contract, Delivery Volume | MW |  |  |  | 12.5 | 12.5 |
Sensitivity analysis in derivative, forward price, percent | 20.00% |  |  |  |  |
Long-term contracts | Canada | Fixed-price contracts [member] | Maximum |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Forward power price ranges per MWh |  | CAD 93 |  | CAD 67 |  |
Long-term contracts | Canada | Fixed-price contracts [member] | Minimum |  |  |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |  |  |
Forward power price ranges per MWh |  | CAD 68 |  | CAD 63 |  |

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