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Summary of significant accounting policies - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
 [2]
Accounting Policies [Abstract]        
Cash and cash equivalents $ 733.9 $ 370.6 $ 365.2  
Restricted cash 30.2 30.5 [1]    
Total cash, cash equivalents and restricted cash shown in the consolidated statements of cash flows $ 764.1 $ 401.1 [2] $ 396.8 [2] $ 530.2
[1] Amounts were changed to conform to the current-year presentation. See Note 5
[2] Amounts were changed to conform to the current-year presentation. See Note 5