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Summary of significant accounting policies - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Accounting Policies [Abstract]        
Cash and cash equivalents $ 538.8 $ 605.0 $ 733.9  
Restricted cash 13.0 19.1    
Total cash, cash equivalents and restricted cash shown in the consolidated statements of cash flows $ 551.8 $ 624.1 $ 764.1 $ 401.1