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Investments - Change in investments (Details) $ in Millions, $ in Millions				12 Months Ended
	Nov. 30, 2020 USD ($)	Nov. 25, 2020 CAD ($)	Nov. 25, 2020 USD ($)	Dec. 31, 2020 CAD ($)	Dec. 31, 2019 CAD ($)	Dec. 31, 2018 CAD ($)
Change In Investments [Roll Forward]
Equity income				$ 1	$ 0	$ 0
Change in foreign exchange rates				17	(15)	$ (15)
Consolidated structured entities [member]
Change In Investments [Roll Forward]
Balance, Dec. 31, 2019				0
Contributions				102
Equity income				1
Change in foreign exchange rates				(3)
Balance, Dec. 31, 2020				100	0
Skookumchuck
Change In Investments [Roll Forward]
Contributions		$ 86	$ 66
EMG International, LLC
Change In Investments [Roll Forward]
Contributions	$ 12
SP Skookumchuck Investment, LLC
Change In Investments [Roll Forward]
Balance, Dec. 31, 2019				0
Contributions				86
Equity income				1
Change in foreign exchange rates				(2)
Balance, Dec. 31, 2020				85	0
EMG International, LLC
Change In Investments [Roll Forward]
Balance, Dec. 31, 2019				0
Contributions				16
Equity income				0
Change in foreign exchange rates				(1)
Balance, Dec. 31, 2020				$ 15	$ 0

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The amount of exchange differences recognised in profit or loss that arise from foreign currency transactions, excluding those arising on financial instruments measured at fair value through profit or loss in accordance with IFRS 9. [Refer: At fair value [member]; Financial instruments, class [member]]

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