| Geographic information - Operations by Geographic Area - Long-lived Assets (Details) - USD ($) $ in Thousands | Dec. 31, 2023 | Dec. 31, 2022 | 
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| Operations by Geographic Area | ||
| Long-lived assets | $ 71,708 | $ 71,535 | 
| U.K. | ||
| Operations by Geographic Area | ||
| Long-lived assets | 40,988 | 42,387 | 
| U.S. | ||
| Operations by Geographic Area | ||
| Long-lived assets | $ 30,720 | $ 29,148 | 
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- References No definition available. 
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- Definition Long-lived assets other than financial instruments, long-term customer relationships of a financial institution, mortgage and other servicing rights, deferred policy acquisition costs, and deferred tax assets. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
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