Income Taxes - Expected Income Tax Expense at Canadian Statutory Rates Reconciled to Income Tax Expense (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Statutory federal and provincial income tax rate (Canada)	26.31%	26.77%	26.86%
Expected income tax expense at Canadian enacted statutory tax rates	$ 842	$ 842	$ 695
(Gains) losses not subject to tax	(23)	(19)	8
Canadian tax rate differentials	(3)	0	0
Foreign tax rate differentials	(32)	(33)	(55)
Effect of tax rate decrease	(32)	(95)	(21)
Valuation allowance	0	(5)	5
Unrecognized tax benefits	(7)	33	0
Other	13	(17)	5
Income tax expense	$ 758	$ 706	$ 637