General and Administrative - Summary of General and Administrative Expenses (Detail) - CAD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
General And Administrative Expenses [Abstract]
Salaries and Benefits	$ 52	$ 35	$ 201	$ 105
Administrative and Other	51	19	159	65
Stock-Based Compensation Expense (Recovery) (Note 24)	28	(3)	97	(15)
Other Long-Term Incentive Benefits Expense (Recovery)	27	0	34	(31)
General and Administrative Expenses	$ 158	$ 51	$ 491	$ 124	$ 292	$ 331	$ 1,020

The expense of all forms of consideration given by an entity in exchange for a service rendered by employees or for the termination of employment.

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