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Rental expense and commitments - Schedule of Rental Expense and Income from Subleases (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]      
Rental expense, less income from subleases $ 71.7 $ 77.4 $ 80.6
Income from subleases $ 4.4 $ 7.8 $ 9.1