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Segment information (Details 3) | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Customer A |  |  |
Customers accounting for more than 5% of the Group's revenue | 9% | 19% | 15%
Customer B |  |  |
Customers accounting for more than 5% of the Group's revenue | 8% | 0% | 0%
Customer C |  |  |
Customers accounting for more than 5% of the Group's revenue | 6% | 10% | 0%
Customer D |  |  |
Customers accounting for more than 5% of the Group's revenue | 0% | 5% | 0%
Customer E |  |  |
Customers accounting for more than 5% of the Group's revenue | 0% | 0% | 7%

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- Definition Description of risks that arise due to the volume of business transacted with a particular customer. At a minimum, the description informs financial statement users of the general nature of the risk, but excludes "Information about Major Customers" that may be disclosed elsewhere (for instance, segment disclosures). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 18 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6351-108592 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 16 -URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6327-108592 + Details
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