Segment information (Details 2)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplier A
Supplier accounting for more than 5% of Group's purchases	55%	45%	63%
Supplier B
Supplier accounting for more than 5% of Group's purchases	8%	10%	7%
Supplier C
Supplier accounting for more than 5% of Group's purchases	7%	9%	5%
Supplier D
Supplier accounting for more than 5% of Group's purchases	7%	4%	5%

Description of risks that arise due to the volume of business transacted with a particular supplier or reliance placed on that supplier. At a minimum, the description informs financial statement users of the general nature of the risk.

Reference 1: 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 2: 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 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=99380358&loc=d3e7102-108593

Reference 4: 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

Name:	us-gaap_ConcentrationRiskSupplier
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration