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Segment reporting - Major customers (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
USD ($)
customer
Dec. 31, 2017
USD ($)
customer
Dec. 31, 2016
USD ($)
customer
Disclosure of major customers [abstract]      
Sales $ 2,274,038 $ 1,741,693 [1] $ 1,576,037 [1]
Number of major customers 0    
Top Ten Customers [Member]      
Disclosure of major customers [abstract]      
Sales $ 758,894 $ 820,897 $ 656,907
Number of major customers | customer 10 10 10
Dow Corning Corporation [Member]      
Disclosure of major customers [abstract]      
Percentage of Company's sales   12.20% 13.70%
[1] The amounts for 2016 have been re-presented to show the results of the Spanish energy business within profit (loss) from continuing operations, as described in Note 1 to the consolidated financial statements.