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Goodwill and intangible assets - Estimation of remaining amortization expense (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Amortization of intangible assets      
2020 $ 15.8    
2021 15.8    
2022 14.6    
2023 13.8    
2024 13.2    
2025 and thereafter 84.5    
Total remaining amortization expense $ 157.7 $ 173.5 $ 189.3
Minimum      
Amortization of intangible assets      
Intangible asset useful life 2 years 3 months 18 days    
Maximum      
Amortization of intangible assets      
Intangible asset useful life 19 years 10 months 24 days