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Income taxes - Tax losses expiring (Details) - USD ($)
$ in Millions
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Tax loss carry-forwards      
Tax loss carry-forwards $ 264.0 $ 228.5 $ 588.2
U.S. operations | 2021      
Tax loss carry-forwards      
Tax loss carry-forwards $ 38.5