Income Taxes - Summary of Income Tax Expense (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Tax Disclosure [Abstract]
Current income tax expense	$ 85	$ 114	$ 166	$ 195
Deferred income tax expense	37	24	78	91
Income tax expense	$ 122	$ 138	$ 244	$ 286