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Deferred income taxes - Reconciliation of changes in deferred tax liability (asset) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Reconciliation of changes in deferred tax liability (asset) [abstract]      
Deferred tax liability (asset), beginning balance $ 11.7 $ 12.1  
Credited/(charged) to consolidated income statement 4.8 (2.3)  
Credited to other comprehensive income (5.8) 4.3  
Credited to equity 0.6   $ (0.3)
Exchange difference 1.3 (2.4)  
Deferred tax liability (asset), ending balance 12.6 11.7 12.1
Accelerated tax depreciation      
Reconciliation of changes in deferred tax liability (asset) [abstract]      
Deferred tax liability (asset), beginning balance (10.9) (11.0)  
Credited/(charged) to consolidated income statement 5.2 0.1  
Credited to other comprehensive income 0.0 0.0  
Credited to equity 0.0    
Exchange difference 0.0 0.0  
Deferred tax liability (asset), ending balance (5.7) (10.9) (11.0)
Other temporary differences      
Reconciliation of changes in deferred tax liability (asset) [abstract]      
Deferred tax liability (asset), beginning balance 2.8 5.1  
Credited/(charged) to consolidated income statement (1.0) (2.1)  
Credited to other comprehensive income (0.6) 0.0  
Credited to equity 0.6    
Exchange difference 0.2 (0.2)  
Deferred tax liability (asset), ending balance 2.0 2.8 5.1
Tax losses      
Reconciliation of changes in deferred tax liability (asset) [abstract]      
Deferred tax liability (asset), beginning balance 5.1 4.7  
Credited/(charged) to consolidated income statement 0.0 0.9  
Credited to other comprehensive income 0.0 0.0  
Credited to equity 0.0    
Exchange difference 0.3 (0.5)  
Deferred tax liability (asset), ending balance 5.4 5.1 4.7
Retirement benefit obligations      
Reconciliation of changes in deferred tax liability (asset) [abstract]      
Deferred tax liability (asset), beginning balance 14.7 13.3  
Credited/(charged) to consolidated income statement 0.6 (1.2)  
Credited to other comprehensive income (5.2) 4.3  
Credited to equity 0.0    
Exchange difference 0.8 (1.7)  
Deferred tax liability (asset), ending balance $ 10.9 $ 14.7 $ 13.3