Fair value measurements - Non-financial assets measured at fair value on a non-recurring basis (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of property, plant and equipment	$ 0	$ (1,007,000)	$ 0	$ (1,211,000)
Impairment of goodwill of discontinued operations		0	(9,796,000)	0
Property, plant and equipment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of property, plant and equipment		(1,007,000)		(1,211,000)
Fair value measurements, non-recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, plant and equipment, fair value disclosure		0		2,488,000
Goodwill of discontinued operations, fair value disclosure			10,050,000
Fair value measurements, non-recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, plant and equipment, fair value disclosure		0		0
Goodwill of discontinued operations, fair value disclosure			0
Fair value measurements, non-recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, plant and equipment, fair value disclosure		0		0
Goodwill of discontinued operations, fair value disclosure			0
Fair value measurements, non-recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, plant and equipment, fair value disclosure		$ 0		$ 2,488,000
Goodwill of discontinued operations, fair value disclosure			$ 10,050,000