Financial and derivative instruments - Summary of estimated fair value of financial instruments (Detail) - CAD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Assets
Derivative Asset	$ 84	$ 59
Liabilities
Derivative liabilities	84	82
Effect of counterparty netting
Assets
Derivative Asset	(34)	(38)
Liabilities
Derivative liabilities	(34)	(38)
Effect of collateral netting
Assets
Derivative Asset	(6)	0
Liabilities
Derivative liabilities	0	(14)
Net carrying value
Assets
Derivative Asset	44	21
Liabilities
Derivative liabilities	50	30
Level 1
Assets
Derivative Asset	40	38
Liabilities
Derivative liabilities	34	52
Level 2
Assets
Derivative Asset	44	21
Liabilities
Derivative liabilities	50	30
Level 3
Assets
Derivative Asset	0	0
Liabilities
Derivative liabilities	$ 0	$ 0

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Name:	us-gaap_DerivativeAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant