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Related party transactions (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Jun. 30, 2016
Dec. 31, 2017
Dec. 31, 2016
Related party transactions      
Salaries and other benefits   $ 9.4 $ 8.2
Post-employment benefits   1.5 0.5
Share-based payments   5.2 16.8
Total compensation   16.1 $ 25.5
Executive compensation settlement to a related party $ 14.2 $ 14.2  
Minimum      
Related party transactions      
Lump-sum payment term   2 years 6 months  
Maximum      
Related party transactions      
Lump-sum payment term   3 years