Restricted net assets (Details) - CNY (¥) ¥ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Percentage of after-tax income required to be transferred to statutory general reserve fund	10.00%
Reserve level threshold for mandatory appropriation requirement (as a percent)	50.00%
Restricted net assets	¥ 3,559,861	¥ 2,678,921

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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