Restricted net assets (Details) - PRC - USD ($)	12 Months Ended
Dec. 31, 2019	Jun. 30, 2020
Restricted net assets
Minimum required percentage of statutory reserves		10.00%
Percentage of statutory reserves to respective registered capital		50.00%
Statutory reserves	$ 0
Restricted amount by subsidiaries	$ 155,858,000	$ 205,858,000

Amount of restricted net assets of consolidated and unconsolidated subsidiaries as of the end of the most recently completed fiscal year.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(e)(3)(ii))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Name:	us-gaap_AmountOfRestrictedNetAssetsForConsolidatedAndUnconsolidatedSubsidiaries
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant