Income taxes - Reconciliation of Uncertain Tax Positions Related to Unrecognized Tax Benefits (Details) - CAD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2022	Dec. 31, 2019	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Reconciliation of Uncertain Tax Positions to Unrecognized Tax Benefits [Roll Forward]
Unrecognized tax benefits at January 1			$ 49	$ 55	$ 52
Tax benefits related to the current year			1	0	0
Tax benefits related to prior years		$ 24	0	0	10
Gross uncertain tax benefits related to prior years	$ (24)		(30)	(6)	(9)
Settlements with taxing authorities			0	0	2
Unrecognized tax benefits at December 31	$ 20	$ 52	$ 20	$ 49	$ 55