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Financial Instruments Sensitivity ranges for the base fair values (Details) number in Millions, CAD in Millions	12 Months Ended
	Dec. 31, 2017 CAD	Dec. 31, 2016 CAD
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Base fair value	CAD 3,664	CAD 4,311
Long-Term Power Sale - U.S. | Long-term contracts
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Base fair value	853	907
Long-Term Power Sale - Alberta | Long-term contracts
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Base fair value	(1)	(3)
Unit Contingent Power Purchases
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Base fair value	44	13
Structured Products - Eastern U.S.
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Base fair value	17	24
Others
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Base fair value	CAD 5	CAD 6
Maximum | Long-Term Power Sale - U.S. | 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, financial instruments	(130)	(76)
Maximum | Long-Term Power Sale - Alberta | 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, financial instruments	(2)	(5)
Maximum | Unit Contingent Power Purchases
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption, financial instruments	(7)	(2)
Maximum | Structured Products - Eastern U.S.
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption, financial instruments	(8)	(8)
Maximum | Others
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption, financial instruments	(9)	(3)
Minimum | Long-Term Power Sale - U.S. | 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, financial instruments	130	69
Minimum | Long-Term Power Sale - Alberta | 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, financial instruments	2	5
Minimum | Unit Contingent Power Purchases
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption, financial instruments	9	4
Minimum | Structured Products - Eastern U.S.
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption, financial instruments	7	8
Minimum | Others
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Sensitivity ranges for the base fair value, reasonably possible change in risk assumption, financial instruments	9	3

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- Definition

Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table.

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The nominal or face amount of a financial instrument, used to calculate payments made on that instrument.

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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Sensitivity Ranges For The Base Fair Value, Reasonably Possible Increase (Decrease) In Risk Assumption, Financial Instruments

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