Income Taxes - Summary of Income Tax Expense (Details) - CAD CAD in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Current income tax expense	CAD 93	CAD 73	CAD 288	CAD 177
Deferred income tax expense	77	50	168	233
Income tax expense	CAD 170	CAD 123	CAD 456	CAD 410