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Share-based awards - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions |  |  |  | 9 Months Ended | 77 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Sep. 30, 2025 | Oct. 25, 2024 | May 11, 2015 | Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | May 16, 2019
Employee Stock Option |  |  |  |  |  |  |
Compensation Related Costs Share Based Payments Disclosure [Line Items] |  |  |  |  |  |  |
Number of shares exercisable (in shares) | 424,978 |  |  | 424,978 |  | 424,978 |
Weighted average contractual term of options exercisable | 2 years 10 months 6 days |  |  |  |  |  |
Weighted average exercise price of options exercisble (USD per share) | $ 152.58 |  |  | $ 152.58 |  | $ 152.58 |
Restricted stock units 2013 |  |  |  |  |  |  |
Compensation Related Costs Share Based Payments Disclosure [Line Items] |  |  |  |  |  |  |
Additional ordinary shares which have been reserved for issuance (in shares) |  | 2,500,000 | 2,500,000 |  |  |  |
Ordinary shares which have been reserved for issuance (in shares) |  | 6,600,000 |  |  |  |  |
Consultants Restricted Stock Units 2019 |  |  |  |  |  |  |
Compensation Related Costs Share Based Payments Disclosure [Line Items] |  |  |  |  |  |  |
Ordinary shares which have been reserved for issuance (in shares) |  |  |  |  |  |  | 250,000
Consultants Restricted Stock Units 2019 | Non-executive directors |  |  |  |  |  |  |
Compensation Related Costs Share Based Payments Disclosure [Line Items] |  |  |  |  |  |  |
Shares vesting period |  |  |  |  |  | 12 months |
Restricted Stock Units (RSUs) |  |  |  |  |  |  |
Compensation Related Costs Share Based Payments Disclosure [Line Items] |  |  |  |  |  |  |
Fair value of stock units vested |  |  |  | $ 9.1 | $ 25.6 |  |
Fair value of stock units granted |  |  |  | 151.9 | 78.2 |  |
Restricted Stock Units (RSUs) | Within next 12 months |  |  |  |  |  |  |
Compensation Related Costs Share Based Payments Disclosure [Line Items] |  |  |  |  |  |  |
Fair value of stock units granted |  |  |  | 73.9 | 1.5 |  |
Performance Share Unit (PSUs) |  |  |  |  |  |  |
Compensation Related Costs Share Based Payments Disclosure [Line Items] |  |  |  |  |  |  |
Fair value of stock units vested |  |  |  | $ 1.9 | $ 1.8 |  |
PSUs based on service and EPS targets | Maximum |  |  |  |  |  |  |
Compensation Related Costs Share Based Payments Disclosure [Line Items] |  |  |  |  |  |  |
Stock units to be granted (in shares) | 62,427 |  |  | 62,427 |  | 62,427 |

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- Definition Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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- Definition Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (d)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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