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Commitments and contingencies (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
plaintiff
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Disclosure of finance lease and operating lease by lessee [line items]      
Minimum lease payments under operating leases recognized in the consolidated income statement $ 5.1 $ 4.8 $ 5.6
Minimum lease payments payable under non-cancellable operating lease 34.4 27.1 30.8
Capital commitments $ 0.6 3.6 3.1
Number of plaintiffs | plaintiff 3    
Leasehold land and buildings | Minimum      
Disclosure of finance lease and operating lease by lessee [line items]      
Useful lives of property, plant and equipment, period 2 years    
Leasehold land and buildings | Maximum      
Disclosure of finance lease and operating lease by lessee [line items]      
Useful lives of property, plant and equipment, period 65 years    
Plant and equipment held under operating leases | Minimum      
Disclosure of finance lease and operating lease by lessee [line items]      
Useful lives of property, plant and equipment, period 2 years    
Plant and equipment held under operating leases | Maximum      
Disclosure of finance lease and operating lease by lessee [line items]      
Useful lives of property, plant and equipment, period 5 years    
Within one year      
Disclosure of finance lease and operating lease by lessee [line items]      
Minimum lease payments payable under non-cancellable operating lease $ 5.4 4.6 4.9
In two to five years      
Disclosure of finance lease and operating lease by lessee [line items]      
Minimum lease payments payable under non-cancellable operating lease 14.5 11.8 13.5
In over five years      
Disclosure of finance lease and operating lease by lessee [line items]      
Minimum lease payments payable under non-cancellable operating lease $ 14.5 $ 10.7 $ 12.4