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Financial instruments - Narrative (Details)
£ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
T
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2017
GBP (£)
T
Dec. 31, 2016
GBP (£)
Disclosure of detailed information about borrowings [line items]          
Book value $ 178,400,000 $ 176,000,000      
Borrowing costs 2,500,000 1,800,000      
Cash held in escrow 700,000 0      
Exchange differences on translation of foreign operations 11,600,000 (13,100,000) $ (8,600,000)    
Bank and other loans 108,800,000 121,000,000 131,600,000    
Committed Banking Facilities          
Disclosure of detailed information about borrowings [line items]          
Line of credit facility, maximum borrowing capacity 150,000,000 150,000,000      
Bank and other loans 21,300,000 32,800,000      
Letters of credit          
Disclosure of detailed information about borrowings [line items]          
Line of credit facility, maximum borrowing capacity 9,500,000 8,600,000   £ 7.0 £ 7.0
Bank and other loans 0 0      
Bonding Facility          
Disclosure of detailed information about borrowings [line items]          
Line of credit facility, maximum borrowing capacity 4,100,000 3,700,000   3.0 3.0
Bank and other loans 1,400,000 1,300,000   £ 1.0 £ 1.0
Forward foreign currency exchange rate contracts          
Disclosure of detailed information about borrowings [line items]          
Gain (loss) on hedge recognized in equity (700,000) 3,100,000 (400,000)    
Gain (loss) on hedge recognized in profit $ 600,000 900,000 (100,000)    
LME derivative contracts          
Disclosure of detailed information about borrowings [line items]          
Contract totals | T 3,000     3,000  
Gain (loss) on hedge recognized in equity $ (1,200,000) 600,000 3,700,000    
Foreign currency translation risk disclosures          
Disclosure of detailed information about borrowings [line items]          
Gain (loss) on hedge recognized in equity   13,100,000 $ 8,600,000    
Bank and other loan notes          
Disclosure of detailed information about borrowings [line items]          
Book value 111,300,000 122,800,000      
Bank and other loan notes | USD          
Disclosure of detailed information about borrowings [line items]          
Book value $ 105,500,000 $ 117,000,000