Financial and derivative instruments - Summary of estimated fair value of financial instruments (Detail) - CAD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Assets
Derivative Asset	$ 197	$ 59
Liabilities
Derivative liabilities	136	32
Effect of counterparty netting
Assets
Derivative Asset	(36)	(18)
Liabilities
Derivative liabilities	(36)	(18)
Effect of collateral netting
Assets
Derivative Asset	0	(2)
Liabilities
Derivative liabilities	(10)	0
Net carrying value
Assets
Derivative Asset	161	39
Liabilities
Derivative liabilities	90	14
Level 1
Assets
Derivative Asset	36	20
Liabilities
Derivative liabilities	46	18
Level 2
Assets
Derivative Asset	161	39
Liabilities
Derivative liabilities	90	14
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