Income Taxes - Expected Income Tax Expense at Canadian Statutory Rates Reconciled to Income Tax Expense (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Statutory federal and provincial income tax rate (Canada)	26.77%	26.86%	26.56%
Expected income tax expense at Canadian enacted statutory tax rates	$ 842	$ 695	$ 663
(Gains) losses not subject to tax	(19)	8	(27)
Canadian tax rate differentials	0	0	1
Foreign tax rate differentials	(33)	(55)	(9)
Effect of tax rate decrease	(95)	(21)	(541)
Valuation allowance	(5)	5	0
Unrecognized tax benefit	33	0	1
Other	(17)	5	5
Income tax expense	$ 706	$ 637	$ 93