Income Taxes - Movement in net deferred tax assets and liabilities (Details) - CAD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Reconciliation of changes in deferred tax liability (asset) [abstract]
Net deferred tax asset, beginning of year	$ 956,376	$ 1,006,012
Recovery (expense) for the year in net earnings	(18,694)	(52,330)
Recovery (expense) for the year in other comprehensive income	(1,006)	2,754
Effect of movements in exchange rates	2	(60)
Net deferred tax asset, end of year	$ 936,678	$ 956,376

The amount of tax expense or income relating to changes in deferred tax liabilities and deferred tax assets, recognised in profit or loss. [Refer: Deferred tax assets; Deferred tax expense (income); Deferred tax liabilities]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-Subparagraph g
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- Definition

The amount of deferred tax liabilities or assets. [Refer: Deferred tax liabilities; Deferred tax assets]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 12
-IssueDate 2020-01-01
-Paragraph 81
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- Definition

The amount of income tax relating to amounts recognised in other comprehensive income. [Refer: Other comprehensive income]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 12
-IssueDate 2020-01-01
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

The increase (decrease) in deferred tax liability (asset) resulting from the net exchange differences arising when the financial statements are translated from the functional currency into a different presentation currency, including the translation of a foreign operation into the presentation currency of the reporting entity. [Refer: Deferred tax liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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-IssueDate 2020-01-01
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No definition available.

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