| Intangible assets, net - Amortization Expense (Details) - USD ($) | 12 Months Ended | ||
|---|---|---|---|
| Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | |
| Intangible assets, net | |||
| Amortization expense | $ 622,000 | $ 391,000 | $ 160,000 | 
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- Definition The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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| X | ||||||||||
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- References No definition available. 
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