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Income taxes - Reconciliation of unrecognized tax benefits (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Income Tax Disclosure    
Beginning Balance $ 2.5 $ 3.5
Reduction as a result of lapse of statute of limitations   (1.0)
Reduction as a result of change in tax position (1.0)  
Ending Balance $ 1.5 $ 2.5