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Capitalized contract costs (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Capitalized Contract Cost [Line Items]      
Costs to fulfill a contract - amortization expense $ 27.5 $ 24.2 $ 21.7
Costs to fulfill a contract 74.4 75.8  
Deferred Commissions      
Capitalized Contract Cost [Line Items]      
Deferred commissions 8.7 9.1  
Costs to fulfill a contract - amortization expense $ 3.2 $ 3.1 $ 6.5