Fair Value Measurements and Financial Instruments - Fair Value Hierarchy (Details) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Assets
Available-for-sale investment (Note 9 and 28)	CAD 0	CAD 33
Recurring
Assets
Total gross assets	102	63
Less: Counterparty netting not offset on the balance sheet	(9)	(6)
Total net assets	93	57
Liabilities
Total gross liabilities	38	83
Less: Counterparty netting not offset on the balance sheet	(9)	(6)
Total net liabilities	29	77
Recurring \| Not designated as hedging instrument \| Energy contracts subject to regulatory deferral
Assets
Derivative asset	19	7
Liabilities
Derivative liability	26	78
Recurring \| Level 1
Assets
Available-for-sale investment (Note 9 and 28)	0	33
Other investments	69	12
Recurring \| Level 1 \| Not designated as hedging instrument \| Energy contracts subject to regulatory deferral
Assets
Derivative asset	1
Liabilities
Derivative liability		1
Recurring \| Level 2
Assets
Assets held for sale	0	9
Recurring \| Level 2 \| Not designated as hedging instrument \| Energy contracts subject to regulatory deferral
Assets
Derivative asset	13	2
Liabilities
Derivative liability	21	52
Recurring \| Level 2 \| Not designated as hedging instrument \| Energy contracts not subject to regulatory deferral
Liabilities
Derivative liability	9	0
Recurring \| Level 2 \| Designated as hedging instrument \| Interest rate swaps
Assets
Derivative asset	11	0
Liabilities
Derivative liability	3	5
Recurring \| Level 3 \| Not designated as hedging instrument \| Energy contracts subject to regulatory deferral
Assets
Derivative asset	5	5
Liabilities
Derivative liability	5	25
Recurring \| Level 3 \| Not designated as hedging instrument \| Energy contracts not subject to regulatory deferral
Assets
Derivative asset	CAD 3	CAD 2

Assets, Fair Value Disclosure, Net of Amount Not Offset Against Collateral

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Liabilities, Fair Value Disclosure, Net of Amount Not Offset Against Collateral

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Other Investments, Fair Value Disclosure

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Fair value portion of assets classified as held for sale.

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Investments in debt and equity securities which are categorized neither as held-to-maturity nor trading and which are intended to be sold or mature more than one year from the balance sheet date or operating cycle, if longer. Such securities are reported at fair value; unrealized gains (losses) related to Available-for-sale Securities are excluded from earnings and reported in a separate component of shareholders' equity (other comprehensive income), unless the Available-for-sale security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding gain (loss) of an Available-for-sale security that is designated as being hedged in a fair value hedge is recognized in earnings during the period of the hedge, as are other than temporary declines in fair value below the cost basis for investments in equity securities and debt securities that an entity intends to sell or it is more likely than not that it will be required to sell before the recovery of its amortized cost basis. Other than temporary declines in fair value below the cost basis for debt securities categorized as Available-for-sale that an entity does not intend to sell and for which it is not more likely than not that the entity will be required to sell before the recovery of its amortized cost basis are bifurcated into credit losses and losses related to all other factors. Other than temporary declines in fair value below cost basis related to credit losses are recognized in earnings, and losses related to all other factors are recognized in other comprehensive income.

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Fair value of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, elected not to be offset, deducted from derivative assets.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Fair value of financial and nonfinancial obligations.

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