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Income taxes - Reconciliation of Changes in Unrecognized Tax Benefits (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]      
Balance at January 1 $ 35.8 $ 27.7 $ 35.0
Additions based on tax positions related to the current year 4.2 2.7 3.4
Changes for tax positions of prior years (11.2) 12.0 (4.0)
Reductions as a result of a lapse of applicable statute of limitations (2.7) (2.8) (3.4)
Settlements (0.2) (0.1) (0.9)
Changes due to foreign currency 2.0 3.7 2.4
Balance at December 31 $ 27.9 $ 35.8 $ 27.7