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Income taxes - Cash Paid for Income Taxes, Net of Refunds (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Income Tax Disclosure [Abstract]      
Cash paid for income taxes, net of refunds $ 24.7 $ 37.6 $ 39.1