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