Fair Value of Financial Instruments and Risk Management - Derivative Contracts Under Master Netting Agreements and Collateral Positions (Details) (Details) - Energy contracts - CAD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Derivative assets
Gross Amount Recognized in Balance Sheet	$ 44	$ 30
Counterparty Netting of Energy Contracts	30	22
Cash Collateral Received/Posted	10	10
Net Amount	4	(2)
Derivative liabilities
Gross Amount Recognized in Balance Sheet	(125)	(151)
Counterparty Netting of Energy Contracts	(30)	(22)
Cash Collateral Received/Posted	0	(2)
Net Amount	$ (95)	$ (127)

Amount of obligation to return cash collateral under master netting arrangements offset against derivative assets.

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Fair value of liability associated with 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.

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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, before 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 elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Fair value, after effects of master netting arrangements, 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 offset against an obligation to return collateral. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, after effects of master netting arrangements, of 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 offset against the right to receive collateral. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Amount of right to receive cash collateral under master netting arrangements offset against derivative liabilities.

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Fair value of 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, elected not to be offset, deducted from derivative liabilities.

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