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Employee benefits - Narrative (Details) | 12 Months Ended
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Sep. 30, 2024 CAD ($) plan | Sep. 30, 2023 CAD ($)
Disclosure of defined benefit plans [line items] |  |
Surplus in plan | $ (167,920,000) | $ (162,543,000)
Current service cost | 14,165,000 | 13,734,000
Annual contributions | 8,616,000 |
Pension cost for defined contribution plans | 296,470,000 | 282,284,000
Deferred compensation plan liabilities | 124,447,000 | 97,745,000
Deferred compensation plan assets | $ 112,270,000 | 88,076,000
U.K. |  |
Disclosure of defined benefit plans [line items] |  |
Number of defined benefit pension plans | plan | 3 |
Surplus in plan | $ 22,230,000 | 593,000
Annual contributions | 369,000 |
Other |  |
Disclosure of defined benefit plans [line items] |  |
Surplus in plan | (32,668,000) | (28,565,000)
Annual contributions | 8,246,000 |
UNITED STATES |  |
Disclosure of defined benefit plans [line items] |  |
Deferred compensation plan assets | $ 112,270,000 | 88,076,000
CMG U.K. Pension Scheme | U.K. |  |
Disclosure of defined benefit plans [line items] |  |
Equity investments, target allocation percentage of plan assets | 65.00% |
Surplus in plan | $ 36,812,000 |
Logica U.K. Pension & Life Assurance Scheme | U.K. |  |
Disclosure of defined benefit plans [line items] |  |
Equity investments, target allocation percentage of plan assets | 15.00% |
Bond investments, target allocation percentage of plan assets | 85.00% |
Surplus in plan | $ 91,000 |
Current service cost | $ 447,000 |
Logica Defined Benefit Pension Plan | U.K. |  |
Disclosure of defined benefit plans [line items] |  |
Equity investments, target allocation percentage of plan assets | 10.00% |
Bond investments, target allocation percentage of plan assets | 90.00% |
Surplus in plan | $ 18,901,000 |
Alecta multi-employer plan |  |
Disclosure of defined benefit plans [line items] |  |
Annual contributions | 18,043,000 |
Pension cost for defined contribution plans | $ 23,422,000 | $ 25,311,000
Proportion of total contributions to the plan | 0.72% |
Proportion of total number of active members in the plan | 0.48% |
Collective funding solvency percentage | 163.00% | 178.00%
Alecta multi-employer plan | Minimum |  |
Disclosure of defined benefit plans [line items] |  |
Collective funding solvency allowed | 125.00% |
Alecta multi-employer plan | Maximum |  |
Disclosure of defined benefit plans [line items] |  |
Collective funding solvency allowed | 175.00% |

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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 -Name IAS -Number 19 -IssueDate 2024-01-01 -Paragraph 141 -Subparagraph a -URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=19&code=ifrs-tx-2024-en-r&anchor=para_141_a&doctype=Standard -URIDate 2024-03-27 + Details
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- Definition The estimate of the expected contributions to be made to a defined benefit plan for the next annual reporting period. [Refer: Defined benefit plans [domain]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2024-01-01 -Paragraph 147 -Subparagraph b -URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=19&code=ifrs-tx-2024-en-r&anchor=para_147_b&doctype=Standard -URIDate 2024-03-27 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2024-01-01 -Paragraph 148 -Subparagraph d -Clause iii -URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=19&code=ifrs-tx-2024-en-r&anchor=para_148_d_iii&doctype=Standard -URIDate 2024-03-27 + Details
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