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Summary of significant accounting policies - Property and equipment, net, Intangible assets and Impairment long-lived assets (Details) - CNY (¥)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Property and equipment, net      
Impairment of long-lived assets ¥ 0 ¥ 0 ¥ 0
Minimum      
Property and equipment, net      
Intangible assets useful life 2 years    
Maximum      
Property and equipment, net      
Intangible assets useful life 10 years    
Computer and transmission equipment      
Property and equipment, net      
Property and equipment useful life 3 years    
Furniture and office equipment      
Property and equipment, net      
Property and equipment useful life 5 years    
Motor vehicles      
Property and equipment, net      
Property and equipment useful life 4 years