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Retirement benefits - Narrative (Details)
£ in Thousands
 1 Months Ended 12 Months Ended
Aug. 31, 2005
GBP (£)
Dec. 31, 2017
USD ($)
year
Member
Dec. 31, 2017
GBP (£)
year
Member
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2015
GBP (£)
Disclosure of defined benefit plans [line items]            
Total charge (credit) for retirement plans   $ (6,600,000)   $ (6,400,000) $ 9,300,000  
Remeasurement of defined benefit retirement plans   9,500,000   (21,700,000) 4,400,000  
Curtailment credit arising from changes to defined benefit pension plans   0   600,000 3,300,000  
Past service credit   0   0 14,900,000  
Changes to defined benefit pension plans   0   600,000 18,000,000  
Remeasurement of defined benefit retirement plans   (9,500,000)   21,700,000    
Return on plan assets, gain (loss)   33,200,000   56,400,000 (600,000)  
Defined benefit obligation   422,800,000   383,400,000    
Estimate of contributions expected to be paid to plan for next annual reporting period   8,700,000        
Actual employer contributions   8,900,000        
Unfunded Plan            
Disclosure of defined benefit plans [line items]            
Defined benefit obligation   2,100,000   2,600,000    
Funded Plan            
Disclosure of defined benefit plans [line items]            
Defined benefit obligation   $ 420,700,000   380,800,000    
U.K.            
Disclosure of defined benefit plans [line items]            
Salary earnings cap on defined benefit plans | £ £ 60         £ 76
Number of active members in employee benefit plan | Member   160 160      
Weighted average duration of defined benefit obligation | year   18 18      
Minimum annual contribution | £     £ 3,800      
Expected percent of earnings rate one   15.00% 15.00%      
Expected percent of earnings rate two   10.00% 10.00%      
Expected earnings amount two | £     £ 24,000      
Total charge (credit) for retirement plans   $ (3,500,000)   (3,800,000) 12,000,000  
Curtailment credit arising from changes to defined benefit pension plans   0   0 3,300,000  
Past service credit   0   0 14,900,000  
Defined benefit obligation   369,700,000   334,800,000    
U.K. | Minimum            
Disclosure of defined benefit plans [line items]            
Expected earnings amount one | £     12,000      
U.K. | Maximum            
Disclosure of defined benefit plans [line items]            
Expected earnings amount one | £     £ 24,000      
Non-U.K.            
Disclosure of defined benefit plans [line items]            
Total charge (credit) for retirement plans   (3,100,000)   (2,600,000) (2,700,000)  
Annuity settlement payments       10,000,000    
Credit (charge) from annuity settlement payments       (100,000)    
Lump sum settlement payments       4,200,000    
Credit (charge) from cash settlement payment       700,000    
Curtailment credit arising from changes to defined benefit pension plans   0   600,000 0  
Past service credit   0   0 0  
Defined benefit obligation   53,100,000   48,600,000    
Cost of sales            
Disclosure of defined benefit plans [line items]            
Total charge (credit) for retirement plans   (4,100,000)   (4,100,000) 4,600,000  
Administrative costs            
Disclosure of defined benefit plans [line items]            
Total charge (credit) for retirement plans   (700,000)   (900,000) 1,300,000  
Changes to defined benefit pension plans            
Disclosure of defined benefit plans [line items]            
Total charge (credit) for retirement plans   0   600,000 18,000,000  
Finance costs            
Disclosure of defined benefit plans [line items]            
Total charge (credit) for retirement plans   $ (1,800,000)   (2,000,000) $ (3,000,000)  
$10 million liabilities settlement of U.S, Plan | Non-U.K.            
Disclosure of defined benefit plans [line items]            
Gross amounts settled       14,800,000    
Settlement lump sums paid | Non-U.K.            
Disclosure of defined benefit plans [line items]            
Gross amounts settled       $ 14,200,000