Supplementary cash flow information - Net change in non-cash working capital (Details) - CAD ($) $ in Thousands	12 Months Ended
Sep. 30, 2025	Sep. 30, 2024
Cash Flow Statement [Abstract]
Accounts receivable	$ 80,109	$ 106,360
Work in progress	(63,386)	(8,999)
Prepaid expenses and other assets	49,158	4,466
Long-term financial assets	(9,329)	(24,423)
Accounts payable and accrued liabilities	(167,764)	22,151
Accrued compensation and employee-related liabilities	(59,885)	(27,689)
Deferred revenue	(1,932)	50,420
Income taxes	(85,430)	(98,207)
Provisions	122,426	(594)
Long-term liabilities	7,739	33,540
Derivative financial instruments	118	634
Retirement benefits obligations	9,133	7,337
Increase (decrease) in working capital	$ (119,043)	$ 64,996

Adjustments For Decrease (Increase) In Non-Current Financial Assets

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- Definition

Adjustments For Decrease (Increase) In Prepayments And Other Current Assets

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Adjustments For Decrease (Increase) In Work In Progress

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- Definition

Adjustments For Increase (Decrease) In Provisions

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- Definition

Adjustments For Increase (Decrease) In Short-Term Employee Benefits Accruals

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- Definition

Adjustments For Increase (Decrease) In Tax Liabilities

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- Definition

Cash Flows Statement [Abstract]

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- Definition

Adjustments for decrease (increase) in loans and advances to banks to reconcile profit (loss) to net cash flow from (used in) operating activities. [Refer: Loans and advances to banks; Profit (loss)]

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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Adjustments for decrease (increase) in trade and other receivables to reconcile profit (loss) to net cash flow from (used in) operating activities. [Refer: Trade and other receivables; Profit (loss)]

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Adjustments for the increase (decrease) in deferred income including contract liabilities to reconcile profit (loss) to net cash flow from (used in) operating activities. [Refer: Deferred income including contract liabilities; Profit (loss)]

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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Adjustments for increase (decrease) in derivative financial liabilities to reconcile profit (loss) to net cash flow from (used in) operating activities. [Refer: Derivative financial liabilities; Profit (loss)]

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Adjustments for increase (decrease) in employee benefit liabilities to reconcile profit (loss) to net cash flow from (used in) operating activities. [Refer: Profit (loss)]

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- Definition

Adjustments for increase (decrease) in trade and other payables to reconcile profit (loss) to net cash flow from (used in) operating activities. [Refer: Trade and other payables; Profit (loss)]

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- Definition

The increase (decrease) in working capital.

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