Principal accounting policies (Sales and Marketing Expenses, Share based Compensation, Statutory Reserves, Dividends and Segment Reporting - Narrative) (Details) - CNY (¥) ¥ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statutory reserves [Line Item]
Advertising and market promotion expenses	¥ 621,771	¥ 298,681	¥ 253,210
Employee social security and welfare benefits	¥ 214,848	206,704	171,349
Minimum percentage appropriation to general reserve fund required	10.00%
Reserve level threshold for mandatory appropriation requirement (as a percent)	50.00%
Minimum percentage appropriation to statutory surplus fund required	10.00%
Surplus fund threshold for mandatory appropriation requirement (as a percent)	50.00%
Dividends
Dividends declared	¥ 0	0	0
Statutory reserves [Member]
Statutory reserves [Line Item]
Amount appropriated to statutory reserves	¥ 3,861	¥ 2,350	¥ 38

Amount of cost for defined contribution plan.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

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Amount of paid and unpaid common stock dividends declared with the form of settlement in cash, stock and payment-in-kind (PIK).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 405
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6957935&loc=d3e64057-112817

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The total expense recognized in the period for promotion, public relations, and brand or product advertising.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.4)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

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Represents Statutory reserves appropriated during the period.

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Represents the level of the general reserve fund balance as a percentage of registered capital for the entity's subsidiaries and the VIEs and VIE''s subsidiary in the PRC, which must be attained before mandatory transfers to the reserve are no longer required.

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