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Fair value measurements - Non-financial assets measured at fair value on a non-recurring basis (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Valuation of financial assets and liabilities by the fair value hierarchy      
Impairment of property, plant and equipment $ (8,216,000) $ (1,007,000) $ 0
Impairment of goodwill of discontinued operations (5,500,000) 0 (9,796,000)
Property, plant and equipment      
Valuation of financial assets and liabilities by the fair value hierarchy      
Impairment of property, plant and equipment (8,216,000) (1,007,000)  
Fair value measurements, non-recurring      
Valuation of financial assets and liabilities by the fair value hierarchy      
Property, plant and equipment, fair value disclosure 5,955,000 0  
Goodwill of discontinued operations, fair value disclosure 0   10,050,000
Fair value measurements, non-recurring | Level 1      
Valuation of financial assets and liabilities by the fair value hierarchy      
Property, plant and equipment, fair value disclosure 0 0  
Goodwill of discontinued operations, fair value disclosure 0   0
Fair value measurements, non-recurring | Level 2      
Valuation of financial assets and liabilities by the fair value hierarchy      
Property, plant and equipment, fair value disclosure 0 0  
Goodwill of discontinued operations, fair value disclosure 0   0
Fair value measurements, non-recurring | Level 3      
Valuation of financial assets and liabilities by the fair value hierarchy      
Property, plant and equipment, fair value disclosure 5,955,000 $ 0  
Goodwill of discontinued operations, fair value disclosure $ 0   $ 10,050,000