Financial Instruments - Carrying Amounts and Reconciliation of Derivative Financial Instruments (Details) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of detailed information about hedging instruments [line items]
Derivative financial asset (liability), net	$ (24)	$ 178	$ 356
Current portion of other long-term assets	5	8
Current portion of other long-term liabilities	(131)	(112)
Other long-term assets	131	282
Other long-term liabilities	(29)	0
Cash flow hedges
Disclosure of detailed information about hedging instruments [line items]
Gain (loss) on hedge ineffectiveness	1	(3)	$ 2
Natural gas fixed price swaps
Disclosure of detailed information about hedging instruments [line items]
Derivatives held for trading, asset (liability)	(5)	(3)
Natural gas basis swaps
Disclosure of detailed information about hedging instruments [line items]
Derivatives held for trading, asset (liability)	(40)	(8)
Foreign currency forward contracts
Disclosure of detailed information about hedging instruments [line items]
Derivatives held for trading, asset (liability)	(7)	(10)
Foreign currency forward contracts \| Cash flow hedges
Disclosure of detailed information about hedging instruments [line items]
Cash flow hedges, asset (liability)	(108)	(91)
Cross currency swaps \| Cash flow hedges
Disclosure of detailed information about hedging instruments [line items]
Cash flow hedges, asset (liability)	$ 136	$ 290

Derivative Financial Assets (Liabilities) Held For Hedging

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- Definition

Derivative Financial Assets (Liabilities) Held For Trading

+ References

No definition available.

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Derivative Financial Assets (Liabilities), Net

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No definition available.

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- Definition

The amount of current derivative financial assets. [Refer: Derivative financial assets]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
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-IssueDate 2020-01-01
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- Definition

The amount of current derivative financial liabilities. [Refer: Derivative financial liabilities]

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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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No definition available.

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The gain (loss) recognised in profit or loss on hedge ineffectiveness. [Refer: Gain (loss) on hedge ineffectiveness]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2020-01-01
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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-IssueDate 2020-01-01
-Paragraph 24C
-Subparagraph b
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- Definition

The amount of non-current derivative financial assets. [Refer: Derivative financial assets]

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The amount of non-current derivative financial liabilities. [Refer: Derivative financial liabilities]

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