Long-term debt - Table Narrative (Detail) - CAD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Debt Disclosure [Abstract]
Maximum long-term borrowing from affiliate		$ 7,750,000,000
Cancellation period long-term borrowing from affiliate, days		370 days
Repayments of long-term debt	$ 1,000,000,000	$ 0	$ 1,000,000,000	$ 0
Finance lease obligations for marine services, average imputed rate		4.70%	4.70%
Finance lease, liability current		$ 21,000,000	$ 22,000,000
Principal payments on finance lease, year two		18,000,000
Principal payments on finance lease, year three		18,000,000
Principal payments on finance lease, year four		18,000,000
Principal payments on finance lease, year five		$ 18,000,000