Income Taxes - Summary of Income Tax Expense (Details) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Current income tax expense	$ 115	$ 122	$ 358	$ 288
Deferred income tax (recovery) expense	96	77	116	155
Income tax expense	$ 211	$ 199	$ 474	$ 443