Principal accounting policies (Sales and Marketing Expenses, Share based Compensation, Statutory Reserves, Dividends and Segment Reporting - Narrative) (Details) ¥ in Thousands	12 Months Ended
Dec. 31, 2016 CNY (¥)	Dec. 31, 2015 CNY (¥)	Dec. 31, 2014 CNY (¥)
Sales and marketing expenses
Advertising and market promotion expenses	¥ 298,681	¥ 253,210	¥ 76,192
Employee social security and welfare benefits
Employee social security and welfare benefits	¥ 206,704	171,349	115,012
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%
Amount appropriated to statutory reserves
Dividends
Dividends declared	¥ 0	0	0
Segment reporting
Number of operating segments	1
Statutory reserves [Member]
Employee social security and welfare benefits
Amount appropriated to statutory reserves	¥ 2,350	¥ 38	¥ 15,812

The amount of the cost recognized during the period for defined contribution plans.

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
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-Name Accounting Standards Codification
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-SubTopic 405
-Section 45
-Paragraph 2
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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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Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

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Represents Statutory reserves appropriated during the period.

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Represents the minimum percentage of after-tax profit required to be appropriated towards the general reserve fund for the entity's subsidiaries registered as wholly-owned foreign enterprises until the reserve balance reaches 50 percent of registered capital.

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Represents the minimum percentage of after-tax profit required to be appropriated towards the statutory surplus fund for the entities incorporated in the People's Republic Of China until the reserve balance reaches 50 percent of registered capital.

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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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Represents the level of the statutory surplus fund balance as a percentage of registered capital for the entities incorporated in the People's Republic of China, which must be attained before mandatory transfers to the fund are no longer required.

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