Financial instruments - Financial liabilities included in long-term debt (Details) - CAD ($) $ in Thousands	Sep. 30, 2025	Sep. 30, 2024
Disclosure of financial liabilities [line items]
Financial liabilities	$ 7,787,320	$ 6,011,388
Carrying amount
Disclosure of financial liabilities [line items]
Financial liabilities	3,637,835	2,688,308
Fair value
Disclosure of financial liabilities [line items]
Financial liabilities	3,599,707	2,549,382
2021 U.S. Senior Notes \| Carrying amount
Disclosure of financial liabilities [line items]
Financial liabilities	1,386,564	1,342,758
2021 U.S. Senior Notes \| Fair value
Disclosure of financial liabilities [line items]
Financial liabilities	1,310,044	1,223,120
2021 CAD Senior Notes \| Carrying amount
Disclosure of financial liabilities [line items]
Financial liabilities	597,892	597,212
2021 CAD Senior Notes \| Fair value
Disclosure of financial liabilities [line items]
Financial liabilities	580,561	564,768
2024 CAD Senior Notes \| Carrying amount
Disclosure of financial liabilities [line items]
Financial liabilities	747,001	746,144
2024 CAD Senior Notes \| Fair value
Disclosure of financial liabilities [line items]
Financial liabilities	766,844	759,375
2025 U.S. Senior Notes \| Carrying amount
Disclosure of financial liabilities [line items]
Financial liabilities	894,509	0
2025 U.S. Senior Notes \| Fair value
Disclosure of financial liabilities [line items]
Financial liabilities	930,366	0
Other long-term debt \| Carrying amount
Disclosure of financial liabilities [line items]
Financial liabilities	11,869	2,194
Other long-term debt \| Fair value
Disclosure of financial liabilities [line items]
Financial liabilities	$ 11,892	$ 2,119

- Definition

The amount of liabilities that are: (a) a contractual obligation: (i) to deliver cash or another financial asset to another entity; or (ii) to exchange financial assets or financial liabilities with another entity under conditions that are potentially unfavourable to the entity; or (b) a contract that will, or may be, settled in the entity’s own equity instruments and is: (i) a non-derivative for which the entity is, or may be, obliged to deliver a variable number of the entity’s own equity instruments; or (ii) a derivative that will, or may be, settled other than by the exchange of a fixed amount of cash or another financial asset for a fixed number of the entity’s own equity instruments. For this purpose, rights, options or warrants to acquire a fixed number of the entity’s own equity instruments for a fixed amount of any currency are equity instruments if the entity offers the rights, options or warrants pro rata to all of its existing owners of the same class of its own non-derivative equity instruments. Also, for those purposes the entity’s own equity instruments do not include puttable financial instruments that are classified as equity instruments in accordance with paragraphs 16A-16B of IAS 32, instruments that impose on the entity an obligation to deliver to another party a pro rata share of the net assets of the entity only on liquidation and are classified as equity instruments in accordance with paragraphs 16C-16D of IAS 32, or instruments that are contracts for the future receipt or delivery of the entity’s own equity instruments. As an exception, an instrument that meets the definition of a financial liability is classified as an equity instrument if it has all the features and meets the conditions in paragraphs 16A-16B or paragraphs 16C-16D of IAS 32. [Refer: Classes of financial instruments [domain]; Financial assets; Derivatives [member]]

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