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Income taxes - Summary of Income Tax Expense (Benefit) (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2020
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Operating Loss Carryforwards [Line Items]                      
Income tax expense (benefit) $ 3,728 $ 652 $ 3,309 $ 419 $ (718) $ (213) $ (74) $ 1,718 $ 8,108 $ 713 $ (4,010)
Income tax expense (benefit) related to U.S. and U.K. subsidiaries                      
Operating Loss Carryforwards [Line Items]                      
Income tax expense (benefit)                 8,040 713 (10,035)
Change in uncertain tax positions                      
Operating Loss Carryforwards [Line Items]                      
Income tax expense (benefit)                 68 0 (300)
Withholding taxes on certain investment transactions (1)                      
Operating Loss Carryforwards [Line Items]                      
Income tax expense (benefit)                 $ 0 $ 0 $ 6,325