Restricted short-term deposits (Narrative) (Details) - Dec. 31, 2017 ¥ in Millions, $ in Millions	CNY (¥)	USD ($)
Restricted Cash And Investments [Line Items]
Debt Instrument, Collateral Amount \| ¥	¥ 1,000
Line of Credit Facility, Maximum Borrowing Capacity \| $		$ 160

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant