Income taxes - Expected Income Tax Expense at Canadian Statutory Rates Reconciled to Income Tax Expense (Details) - CAD ($) $ in Millions	3 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Statutory federal and provincial income tax rate (Canada)		26.12%	26.31%	26.77%
Expected income tax expense at Canadian enacted statutory tax rates		$ 946	$ 842	$ 842
(Decrease) increase in taxes resulting from:
Gains not subject to tax		(116)	(23)	(19)
Canadian tax rate differentials		(22)	(3)	0
Foreign tax rate differentials		(37)	(32)	(33)
Effect of tax rate decrease	$ (88)	(11)	(32)	(95)
Valuation allowance		0	0	(5)
Unrecognized tax benefits		(2)	(7)	33
Other		10	13	(17)
Total income taxes		$ 768	$ 758	$ 706