Income Taxes - Summary of Income Tax Expense (Details) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Current income tax expense	$ 144	$ 115	$ 430	$ 358
Deferred income tax expense	45	96	133	116
Income tax expense	$ 189	$ 211	$ 563	$ 474