| Property, plant and equipment, net - Depreciation Expense (Details) - USD ($) | 12 Months Ended | ||
|---|---|---|---|
| Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | |
| Property, plant and equipment | |||
| Depreciation expense | $ 7,188,000 | $ 5,032,000 | $ 3,126,000 | 
| Leasehold improvements | |||
| Property, plant and equipment | |||
| Loss on disposal | $ 7,000 | $ 194,000 | $ 122,000 | 
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- Definition The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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- Definition Amount of gain (loss) on sale or disposal of property, plant and equipment assets, including oil and gas property and timber property. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. No definition available. 
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- Details 
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