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Concentration and risks (Details) - CNY (¥)
¥ in Thousands
12 Months Ended 36 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Jun. 30, 2008
Concentration and risks      
Foreign currency exchange appreciation (depreciation) rate (5.00%) 7.00%  
Minimum      
Concentration and risks      
Foreign currency exchange appreciation (depreciation) rate     20.00%
Currency convertibility risk      
Concentration and risks      
Cash and cash equivalents, restricted cash and short-term investments ¥ 15,443,123 ¥ 27,566,040