Fair value measurements - Non-financial assets measured at fair value on a non-recurring basis (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Valuation of financial assets and liabilities by the fair value hierarchy
Impairment of property, plant and equipment	$ (4,931,000)	$ (8,216,000)	$ (1,007,000)
Goodwill	111,072,000	120,220,000	113,343,000
Impairment of goodwill of discontinued operations	(4,719,000)	(5,500,000)	0
Other intangible assets	27,141,000	19,778,000
Goodwill and Intangible Asset Impairment	(156,000)
Property, plant and equipment
Valuation of financial assets and liabilities by the fair value hierarchy
Impairment of property, plant and equipment	(4,775,000)	(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	0	5,955,000	0
Goodwill	0	0
Other intangible assets	0
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	0
Goodwill of discontinued operations, fair value disclosure	0	0
Other intangible assets	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	0
Goodwill of discontinued operations, fair value disclosure	0	0
Other intangible assets	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	0	5,955,000	$ 0
Goodwill of discontinued operations, fair value disclosure	0	$ 0
Other intangible assets	$ 0