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Summary of Significant Accounting Policies - Property, equipment and software (Details)
12 Months Ended
Dec. 31, 2019
Computer and office equipment  
Property, Equipment and Software, Net  
Estimated useful life 3 years
Purchased software | Minimum  
Property, Equipment and Software, Net  
Estimated useful life 3 years
Purchased software | Maximum  
Property, Equipment and Software, Net  
Estimated useful life 5 years