Summary of significant accounting policies (Contract Liabilities) (Details) - USD ($) |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2020 |
Dec. 31, 2019 |
Dec. 31, 2018 |
|
| Summary of significant accounting policies | |||
| Unbilled loans mature amounted | $ 0 | $ 0 | |
| Balance | 8,000 | 6,000 | |
| Acquisition of Suns Tower Limited | $ 10,000 | ||
| Revenue recognized from beginning contract liability balance | (8,000) | (6,000) | (10,000) |
| Advances received from customers related to unsatisfied performance obligations | 3,000 | 8,000 | 6,000 |
| Balance | $ 3,000 | $ 8,000 | $ 6,000 |