| Financial instruments - Foreign Exchange Risk (Details) - $ / £ | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | 
|---|---|---|---|
| Financial instruments | |||
| Exchange rate | 1.27 | 1.21 | 1.35 | 
| X | ||||||||||
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- Definition Foreign exchange rate used to translate amounts denominated in functional currency to reporting currency. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
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| X | ||||||||||
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- References No definition available. 
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