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Income taxes - Reconciliation of unrecognized tax benefits (Details) - USD ($)
$ in Millions
Mar. 31, 2022
Dec. 31, 2021
Mar. 31, 2021
Income Tax Disclosure      
Beginning Balance $ 3.5 $ 14.8 $ 14.8
Ending Balance $ 3.5 $ 3.5 $ 14.8