Employee benefits - Defined benefit obligations (Details) - CAD ($) $ in Thousands	12 Months Ended
Sep. 30, 2023	Sep. 30, 2022
Disclosure of net defined benefit liability (asset) [line items]
Defined benefit liability (asset), beginning balance	$ 89,097
Current service cost	13,734	$ 13,053
Interest cost	2,998	51
Past service cost	(288)	0
Defined benefit liability (asset), ending balance	143,085	89,097
U.K.
Disclosure of net defined benefit liability (asset) [line items]
Defined benefit liability (asset), beginning balance	(46,647)
Defined benefit liability (asset), ending balance	(593)	(46,647)
France
Disclosure of net defined benefit liability (asset) [line items]
Defined benefit liability (asset), beginning balance	77,477
Defined benefit liability (asset), ending balance	78,612	77,477
Germany
Disclosure of net defined benefit liability (asset) [line items]
Defined benefit liability (asset), beginning balance	31,897
Defined benefit liability (asset), ending balance	36,877	31,897
Other
Disclosure of net defined benefit liability (asset) [line items]
Defined benefit liability (asset), beginning balance	26,370
Defined benefit liability (asset), ending balance	28,189	26,370
Present value of defined benefit obligation
Disclosure of net defined benefit liability (asset) [line items]
Defined benefit liability (asset), beginning balance	749,943	1,160,956
Current service cost	13,733	13,053
Interest cost	37,552	21,354
Past service cost	(288)
Business acquisitions		10,192
Actuarial losses (gains) due to change in financial assumptions	(60,689)	(342,078)
Actuarial losses (gains) due to change in demographic assumptions	(11,987)	8,283
Actuarial (gains) losses due to experience	33,381	23,089
Plan participant contributions	246	266
Benefits paid from the plan	(31,115)	(31,708)
Benefits paid directly by employer	(6,304)	(5,032)
Foreign currency translation adjustment	50,182	(96,438)
Other		(11,994)
Defined benefit liability (asset), ending balance	774,654	749,943
Present value of defined benefit obligation \| Unfunded Plans
Disclosure of net defined benefit liability (asset) [line items]
Defined benefit liability (asset), beginning balance	96,306
Defined benefit liability (asset), ending balance	96,744	96,306
Present value of defined benefit obligation \| Funded Plans
Disclosure of net defined benefit liability (asset) [line items]
Defined benefit liability (asset), beginning balance	653,637
Defined benefit liability (asset), ending balance	677,910	653,637
Present value of defined benefit obligation \| U.K.
Disclosure of net defined benefit liability (asset) [line items]
Defined benefit liability (asset), beginning balance	525,262	881,008
Current service cost	997	1,114
Interest cost	27,445	16,877
Past service cost	0
Business acquisitions		0
Actuarial losses (gains) due to change in financial assumptions	(54,598)	(285,653)
Actuarial losses (gains) due to change in demographic assumptions	(12,077)	7,882
Actuarial (gains) losses due to experience	33,349	4,081
Plan participant contributions	76	80
Benefits paid from the plan	(26,527)	(24,018)
Benefits paid directly by employer	0	0
Foreign currency translation adjustment	41,706	(76,109)
Other		0
Defined benefit liability (asset), ending balance	535,633	525,262
Present value of defined benefit obligation \| U.K. \| Unfunded Plans
Disclosure of net defined benefit liability (asset) [line items]
Defined benefit liability (asset), beginning balance	0
Defined benefit liability (asset), ending balance	0	0
Present value of defined benefit obligation \| U.K. \| Funded Plans
Disclosure of net defined benefit liability (asset) [line items]
Defined benefit liability (asset), beginning balance	525,262
Defined benefit liability (asset), ending balance	535,633	525,262
Present value of defined benefit obligation \| France
Disclosure of net defined benefit liability (asset) [line items]
Defined benefit liability (asset), beginning balance	77,477	77,006
Current service cost	6,106	5,673
Interest cost	3,093	740
Past service cost	(288)
Business acquisitions		10,192
Actuarial losses (gains) due to change in financial assumptions	(4,575)	(20,586)
Actuarial losses (gains) due to change in demographic assumptions	88	921
Actuarial (gains) losses due to experience	(6,035)	12,112
Plan participant contributions	0	0
Benefits paid from the plan	0	(622)
Benefits paid directly by employer	(2,565)	(1,318)
Foreign currency translation adjustment	5,311	(6,641)
Other		0
Defined benefit liability (asset), ending balance	78,612	77,477
Present value of defined benefit obligation \| France \| Unfunded Plans
Disclosure of net defined benefit liability (asset) [line items]
Defined benefit liability (asset), beginning balance	77,477
Defined benefit liability (asset), ending balance	78,612	77,477
Present value of defined benefit obligation \| France \| Funded Plans
Disclosure of net defined benefit liability (asset) [line items]
Defined benefit liability (asset), beginning balance	0
Defined benefit liability (asset), ending balance	0	0
Present value of defined benefit obligation \| Germany
Disclosure of net defined benefit liability (asset) [line items]
Defined benefit liability (asset), beginning balance	61,420	94,381
Current service cost	379	531
Interest cost	2,600	768
Past service cost	0
Business acquisitions		0
Actuarial losses (gains) due to change in financial assumptions	65	(25,735)
Actuarial losses (gains) due to change in demographic assumptions	0	0
Actuarial (gains) losses due to experience	2,571	2,214
Plan participant contributions	0	0
Benefits paid from the plan	(229)	(647)
Benefits paid directly by employer	(2,992)	(2,848)
Foreign currency translation adjustment	3,892	(7,244)
Other		0
Defined benefit liability (asset), ending balance	67,706	61,420
Present value of defined benefit obligation \| Germany \| Unfunded Plans
Disclosure of net defined benefit liability (asset) [line items]
Defined benefit liability (asset), beginning balance	0
Defined benefit liability (asset), ending balance	0	0
Present value of defined benefit obligation \| Germany \| Funded Plans
Disclosure of net defined benefit liability (asset) [line items]
Defined benefit liability (asset), beginning balance	61,420
Defined benefit liability (asset), ending balance	67,706	61,420
Present value of defined benefit obligation \| Other
Disclosure of net defined benefit liability (asset) [line items]
Defined benefit liability (asset), beginning balance	85,784	108,561
Current service cost	6,251	5,735
Interest cost	4,414	2,969
Past service cost	0
Business acquisitions		0
Actuarial losses (gains) due to change in financial assumptions	(1,581)	(10,104)
Actuarial losses (gains) due to change in demographic assumptions	2	(520)
Actuarial (gains) losses due to experience	3,496	4,682
Plan participant contributions	170	186
Benefits paid from the plan	(4,359)	(6,421)
Benefits paid directly by employer	(747)	(866)
Foreign currency translation adjustment	(727)	(6,444)
Other		(11,994)
Defined benefit liability (asset), ending balance	92,703	85,784
Present value of defined benefit obligation \| Other \| Unfunded Plans
Disclosure of net defined benefit liability (asset) [line items]
Defined benefit liability (asset), beginning balance	18,829
Defined benefit liability (asset), ending balance	18,132	18,829
Present value of defined benefit obligation \| Other \| Funded Plans
Disclosure of net defined benefit liability (asset) [line items]
Defined benefit liability (asset), beginning balance	66,955
Defined benefit liability (asset), ending balance	$ 74,571	$ 66,955

Payments From Employer, Net Defined Benefit Liability (Asset)

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- Definition

The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in demographic assumptions that result in remeasurements of the net defined benefit liability (asset). Demographic assumptions deal with matters such as: (a) mortality; (b) rates of employee turnover, disability and early retirement; (c) the proportion of plan members with dependants who will be eligible for benefits; (d) the proportion of plan members who will select each form of payment option available under the plan terms; and (e) claim rates under medical plans. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2023-01-01
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- Definition

The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in financial assumptions that result in remeasurements of the net defined benefit liability (asset). Financial assumptions deal with items such as: (a) the discount rate; (b) benefit levels, excluding any cost of the benefits to be met by employees, and future salary; (c) in the case of medical benefits, future medical costs, including claim handling costs (ie the costs that will be incurred in processing and resolving claims, including legal and adjuster's fees); and (d) taxes payable by the plan on contributions relating to service before the reporting date or on benefits resulting from that service. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from experience adjustments that result in remeasurements of the net defined benefit liability (asset). Experience adjustments deal with the effects of differences between the previous actuarial assumptions and what has actually occurred. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from contributions to a defined benefit plan by plan participants. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from employee service in the current period. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with members defined in one or many axes of the table.

+ References

No definition available.

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The increase (decrease) in the net defined benefit liability (asset) resulting from business combinations and disposals. [Refer: Business combinations [member]; Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from changes in foreign exchange rates. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from changes that the entity does not separately disclose in the same statement or note. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 19
-IssueDate 2023-01-01
-Paragraph 141
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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from the passage of time. [Refer: Interest expense; Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
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- Definition

The amount of deficit or surplus in a defined benefit plan, adjusted for any effect of limiting a net defined benefit asset to the asset ceiling. [Refer: Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
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-Paragraph 140
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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from past service cost. Past service cost is the change in the present value of the defined benefit obligation for employee service in prior periods, resulting from a plan amendment (the introduction or withdrawal of, or changes to, a defined benefit plan) or a curtailment (a significant reduction by the entity in the number of employees covered by a plan). [Refer: Net defined benefit liability (asset); Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2023-01-01
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-URIDate 2023-03-23

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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from payments from the plan. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
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-URIDate 2023-03-23

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