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Financial instruments - Fair Value (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of fair value measurement of liabilities [line items]      
Liabilities $ 240.3 $ 249.6 $ 266.0
At fair value | Level 3      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 1.0 $ 2.8 $ 2.9
At fair value | Forward foreign currency exchange rate contracts      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 0.7    
At fair value | Forward foreign currency exchange rate contracts | Level 1      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 0.0    
At fair value | Forward foreign currency exchange rate contracts | Level 2      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 0.7    
At fair value | Forward foreign currency exchange rate contracts | Level 3      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 0.0    
At fair value | LME derivative contracts      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities (1.2)    
At fair value | LME derivative contracts | Level 1      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 0.0    
At fair value | LME derivative contracts | Level 2      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities (1.2)    
At fair value | LME derivative contracts | Level 3      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 0.0    
At fair value | Loan Notes due 2018      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 15.3    
At fair value | Loan Notes due 2018 | Level 1      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 0.0    
At fair value | Loan Notes due 2018 | Level 2      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 15.3    
At fair value | Loan Notes due 2018 | Level 3      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 0.0    
At fair value | Loan Notes due 2021      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 25.2    
At fair value | Loan Notes due 2021 | Level 1      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 0.0    
At fair value | Loan Notes due 2021 | Level 2      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 25.2    
At fair value | Loan Notes due 2021 | Level 3      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 0.0    
At fair value | Loan Notes due 2023      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 26.6    
At fair value | Loan Notes due 2023 | Level 1      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 0.0    
At fair value | Loan Notes due 2023 | Level 2      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 26.6    
At fair value | Loan Notes due 2023 | Level 3      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 0.0    
At fair value | Loan Notes due 2026      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 27.1    
At fair value | Loan Notes due 2026 | Level 1      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 0.0    
At fair value | Loan Notes due 2026 | Level 2      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 27.1    
At fair value | Loan Notes due 2026 | Level 3      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 0.0    
At fair value | Revolving credit facility      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 21.3    
At fair value | Revolving credit facility | Level 1      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 0.0    
At fair value | Revolving credit facility | Level 2      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 21.3    
At fair value | Revolving credit facility | Level 3      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 0.0    
At fair value | Deferred contingent consideration      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 0.7    
At fair value | Deferred contingent consideration | Level 1      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 0.0    
At fair value | Deferred contingent consideration | Level 2      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 0.0    
At fair value | Deferred contingent consideration | Level 3      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 0.7    
At fair value | Deferred consideration      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 0.3    
At fair value | Deferred consideration | Level 1      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 0.0    
At fair value | Deferred consideration | Level 2      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities 0.0    
At fair value | Deferred consideration | Level 3      
Disclosure of fair value measurement of liabilities [line items]      
Liabilities $ 0.3