Quantitative and qualitative disclosure about market risk - Geographical area (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues from External Customers and Long-Lived Assets [Line Items]
Total	$ 4,293	$ 5,115	$ 1,570
Hong Kong
Revenues from External Customers and Long-Lived Assets [Line Items]
Total	4,235	$ 5,115	$ 1,570
Australia
Revenues from External Customers and Long-Lived Assets [Line Items]
Total	$ 58

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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+ Details