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Summary of Principal Accounting Policies - Estimated Useful Lives of Property and Equipment (Details)
12 Months Ended
Dec. 31, 2019
Leasehold Improvements  
Property, Plant and Equipment [Line Items]  
Property and equipment, estimated useful lives Shorter of the lease term or expected useful life
Furniture, fixtures, and equipment | Minimum  
Property, Plant and Equipment [Line Items]  
Property and equipment, estimated useful lives 3 years
Furniture, fixtures, and equipment | Maximum  
Property, Plant and Equipment [Line Items]  
Property and equipment, estimated useful lives 5 years
Motor Vehicles  
Property, Plant and Equipment [Line Items]  
Property and equipment, estimated useful lives 5 years
Software | Minimum  
Property, Plant and Equipment [Line Items]  
Property and equipment, estimated useful lives 2 years
Software | Maximum  
Property, Plant and Equipment [Line Items]  
Property and equipment, estimated useful lives 5 years
Building  
Property, Plant and Equipment [Line Items]  
Property and equipment, estimated useful lives 30 years