Income Taxes - Expected Income Tax Expense at Canadian Statutory Rates Reconciled to Income Tax Expense (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Statutory federal and provincial income tax rate (Canada)	26.86%	26.56%	26.65%
Expected income tax expense at Canadian enacted statutory tax rates	$ 695	$ 663	$ 573
Losses (gains) not subject to tax	8	(27)	(23)
Canadian tax rate differentials	0	1	0
Foreign tax rate differentials	(55)	(9)	0
Effect of tax rate decrease	(21)	(541)	0
Valuation Allowance	(5)	0	0
Other	5	6	3
Income tax expense	$ 637	$ 93	$ 553