Income Taxes - Summary of Income Tax Expense (Details) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Current income tax expense	$ 122	$ 93	$ 288	$ 288
Deferred income tax expense	77	77	155	168
Income tax expense	$ 199	$ 170	$ 443	$ 456