Related party transactions - Details of Balances with Related Parties (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Related Party Transactions [Abstract]
Accounts receivable	$ 3,090	$ 1,495
Accounts and other payables	$ 3,750	$ 2,966

For an unclassified balance sheet, amount of receivables arising from transactions with related parties.

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