Income Taxes - Summary of Income Tax Expense (Details) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Current income tax expense	$ 39	$ 144	$ 427	$ 430
Deferred income tax expense	130	45	190	133
Income tax expense	$ 169	$ 189	$ 617	$ 563